[SCSI] libsas: fix sas_unregister_ports vs sas_drain_work

[SCSI] libsas: fix sas_unregister_ports vs sas_drain_work

We need to hold drain_mutex across the unregistration as port down events
queue device removal as chained events, so we need to make sure no other
drainers are active.

[ 1118.673968] WARNING: at kernel/workqueue.c:996 __queue_work+0x11a/0x326()
[ 1118.681982] Hardware name: S2600CP
[ 1118.686193] Modules linked in: isci(-) libsas scsi_transport_sas nls_utf8
ipv6 uinput sg iTCO_wdt iTCO_vendor_support i2c_i801 i2c_core ioatdma dca
sd_mod sr_mod cdrom ahci libahci libata [last unloaded: scsi_transport_sas]
[ 1118.709893] Pid: 6831, comm: rmmod Not tainted 3.2.0-isci+ #1
[ 1118.716727] Call Trace:
[ 1118.719867]  [<ffffffff8103e9f5>] warn_slowpath_common+0x85/0x9d
[ 1118.727000]  [<ffffffff8103ea27>] warn_slowpath_null+0x1a/0x1c
[ 1118.733942]  [<ffffffff81056d44>] __queue_work+0x11a/0x326
[ 1118.740481]  [<ffffffff81056f99>] queue_work_on+0x1b/0x22
[ 1118.746925]  [<ffffffff81057106>] queue_work+0x37/0x3e
[ 1118.753105]  [<ffffffffa0120e05>] ? sas_discover_event+0x55/0x82 [libsas]
[ 1118.761094]  [<ffffffff813217c3>] scsi_queue_work+0x42/0x44
[ 1118.767717]  [<ffffffffa0120e19>] sas_discover_event+0x69/0x82 [libsas]
[ 1118.775509]  [<ffffffffa0120f5b>] sas_unregister_dev+0xc3/0xcc [libsas]
[ 1118.783319]  [<ffffffffa0120fae>] sas_unregister_domain_devices+0x4a/0xc8 [libsas]
[ 1118.792731]  [<ffffffffa0120071>] sas_deform_port+0x60/0x1a6 [libsas]
[ 1118.800339]  [<ffffffffa01201ea>] sas_unregister_ports+0x33/0x44 [libsas]
[ 1118.808342]  [<ffffffffa011f7e5>] sas_unregister_ha+0x41/0x6b [libsas]
[ 1118.816055]  [<ffffffffa0134055>] isci_unregister+0x22/0x4d [isci]
[ 1118.823384]  [<ffffffffa0143040>] isci_pci_remove+0x2e/0x60 [isci]

Reported-by: Jacek Danecki <jacek.danecki@intel.com>
Signed-off-by: Dan Williams <dan.j.williams@intel.com>
Signed-off-by: James Bottomley <JBottomley@Parallels.com>