RDMA/rtrs-srv: Incorporate ib_register_client into rtrs server init

RDMA/rtrs-srv: Incorporate ib_register_client into rtrs server init

The rnbd_server module's communication manager (cm) initialization depends
on the registration of the "network namespace subsystem" of the RDMA CM
agent module. As such, when the kernel is configured to load the
rnbd_server and the RDMA cma module during initialization; and if the
rnbd_server module is initialized before RDMA cma module, a null ptr
dereference occurs during the RDMA bind operation.

Call trace:

  Call Trace:
   ? xas_load+0xd/0x80
   xa_load+0x47/0x80
   cma_ps_find+0x44/0x70
   rdma_bind_addr+0x782/0x8b0
   ? get_random_bytes+0x35/0x40
   rtrs_srv_cm_init+0x50/0x80
   rtrs_srv_open+0x102/0x180
   ? rnbd_client_init+0x6e/0x6e
   rnbd_srv_init_module+0x34/0x84
   ? rnbd_client_init+0x6e/0x6e
   do_one_initcall+0x4a/0x200
   kernel_init_freeable+0x1f1/0x26e
   ? rest_init+0xb0/0xb0
   kernel_init+0xe/0x100
   ret_from_fork+0x22/0x30
  Modules linked in:
  CR2: 0000000000000015

All this happens cause the cm init is in the call chain of the module
init, which is not a preferred practice.

So remove the call to rdma_create_id() from the module init call chain.
Instead register rtrs-srv as an ib client, which makes sure that the
rdma_create_id() is called only when an ib device is added.

Fixes: 9cb837480424 ("RDMA/rtrs: server: main functionality")
Link: https://lore.kernel.org/r/20200907103106.104530-1-haris.iqbal@cloud.ionos.com
Reported-by: kernel test robot <rong.a.chen@intel.com>
Signed-off-by: Md Haris Iqbal <haris.iqbal@cloud.ionos.com>
Reviewed-by: Jack Wang <jinpu.wang@cloud.ionos.com>
Reviewed-by: Leon Romanovsky <leonro@nvidia.com>
Signed-off-by: Jason Gunthorpe <jgg@nvidia.com>