vfio/mdev: Move the compat_class initialization to module init

vfio/mdev: Move the compat_class initialization to module init

The pointer to mdev_bus_compat_class is statically defined at the top
of mdev_core, and was originally (commit 7b96953bc640 ("vfio: Mediated
device Core driver") serialized by the parent_list_lock. The blamed
commit removed this mutex, leaving the pointer initialization
unserialized. As a result, the creation of multiple MDEVs in parallel
(such as during boot) can encounter errors during the creation of the
sysfs entries, such as:

  [    8.337509] sysfs: cannot create duplicate filename '/class/mdev_bus'
  [    8.337514] vfio_ccw 0.0.01d8: MDEV: Registered
  [    8.337516] CPU: 13 PID: 946 Comm: driverctl Not tainted 6.4.0-rc7 #20
  [    8.337522] Hardware name: IBM 3906 M05 780 (LPAR)
  [    8.337525] Call Trace:
  [    8.337528]  [<0000000162b0145a>] dump_stack_lvl+0x62/0x80
  [    8.337540]  [<00000001622aeb30>] sysfs_warn_dup+0x78/0x88
  [    8.337549]  [<00000001622aeca6>] sysfs_create_dir_ns+0xe6/0xf8
  [    8.337552]  [<0000000162b04504>] kobject_add_internal+0xf4/0x340
  [    8.337557]  [<0000000162b04d48>] kobject_add+0x78/0xd0
  [    8.337561]  [<0000000162b04e0a>] kobject_create_and_add+0x6a/0xb8
  [    8.337565]  [<00000001627a110e>] class_compat_register+0x5e/0x90
  [    8.337572]  [<000003ff7fd815da>] mdev_register_parent+0x102/0x130 [mdev]
  [    8.337581]  [<000003ff7fdc7f2c>] vfio_ccw_sch_probe+0xe4/0x178 [vfio_ccw]
  [    8.337588]  [<0000000162a7833c>] css_probe+0x44/0x80
  [    8.337599]  [<000000016279f4da>] really_probe+0xd2/0x460
  [    8.337603]  [<000000016279fa08>] driver_probe_device+0x40/0xf0
  [    8.337606]  [<000000016279fb78>] __device_attach_driver+0xc0/0x140
  [    8.337610]  [<000000016279cbe0>] bus_for_each_drv+0x90/0xd8
  [    8.337618]  [<00000001627a00b0>] __device_attach+0x110/0x190
  [    8.337621]  [<000000016279c7c8>] bus_rescan_devices_helper+0x60/0xb0
  [    8.337626]  [<000000016279cd48>] drivers_probe_store+0x48/0x80
  [    8.337632]  [<00000001622ac9b0>] kernfs_fop_write_iter+0x138/0x1f0
  [    8.337635]  [<00000001621e5e14>] vfs_write+0x1ac/0x2f8
  [    8.337645]  [<00000001621e61d8>] ksys_write+0x70/0x100
  [    8.337650]  [<0000000162b2bdc4>] __do_syscall+0x1d4/0x200
  [    8.337656]  [<0000000162b3c828>] system_call+0x70/0x98
  [    8.337664] kobject: kobject_add_internal failed for mdev_bus with -EEXIST, don't try to register things with the same name in the same directory.
  [    8.337668] kobject: kobject_create_and_add: kobject_add error: -17
  [    8.337674] vfio_ccw: probe of 0.0.01d9 failed with error -12
  [    8.342941] vfio_ccw_mdev aeb9ca91-10c6-42bc-a168-320023570aea: Adding to iommu group 2

Move the initialization of the mdev_bus_compat_class pointer to the
init path, to match the cleanup in module exit. This way the code
in mdev_register_parent() can simply link the new parent to it,
rather than determining whether initialization is required first.

Fixes: 89345d5177aa ("vfio/mdev: embedd struct mdev_parent in the parent data structure")
Reported-by: Alexander Egorenkov <egorenar@linux.ibm.com>
Signed-off-by: Eric Farman <farman@linux.ibm.com>
Reviewed-by: Kevin Tian <kevin.tian@intel.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Tony Krowiak <akrowiak@linux.ibm.com>
Reviewed-by: Jason Gunthorpe <jgg@nvidia.com>
Link: https://lore.kernel.org/r/20230626133642.2939168-1-farman@linux.ibm.com
Signed-off-by: Alex Williamson <alex.williamson@redhat.com>