scsi: ibmvfc: fix WARN_ON during event pool release

While removing an ibmvfc client adapter a WARN_ON like the following
WARN_ON is seen in the kernel log:

WARNING: CPU: 6 PID: 5421 at ./include/linux/dma-mapping.h:541
ibmvfc_free_event_pool+0x12c/0x1f0 [ibmvfc]
CPU: 6 PID: 5421 Comm: rmmod Tainted: G            E     4.17.0-rc1-next-20180419-autotest #1
NIP:  d00000000290328c LR: d00000000290325c CTR: c00000000036ee20
REGS: c000000288d1b7e0 TRAP: 0700   Tainted: G            E      (4.17.0-rc1-next-20180419-autotest)
MSR:  800000010282b033 <SF,VEC,VSX,EE,FP,ME,IR,DR,RI,LE,TM[E]>  CR: 44008828  XER: 20000000
CFAR: c00000000036e408 SOFTE: 1
GPR00: d00000000290325c c000000288d1ba60 d000000002917900 c000000289d75448
GPR04: 0000000000000071 c0000000ff870000 0000000018040000 0000000000000001
GPR08: 0000000000000000 c00000000156e838 0000000000000001 d00000000290c640
GPR12: c00000000036ee20 c00000001ec4dc00 0000000000000000 0000000000000000
GPR16: 0000000000000000 0000000000000000 00000100276901e0 0000000010020598
GPR20: 0000000010020550 0000000010020538 0000000010020578 00000000100205b0
GPR24: 0000000000000000 0000000000000000 0000000010020590 5deadbeef0000100
GPR28: 5deadbeef0000200 d000000002910b00 0000000000000071 c0000002822f87d8
NIP [d00000000290328c] ibmvfc_free_event_pool+0x12c/0x1f0 [ibmvfc]
LR [d00000000290325c] ibmvfc_free_event_pool+0xfc/0x1f0 [ibmvfc]
Call Trace:
[c000000288d1ba60] [d00000000290325c] ibmvfc_free_event_pool+0xfc/0x1f0 [ibmvfc] (unreliable)
[c000000288d1baf0] [d000000002909390] ibmvfc_abort_task_set+0x7b0/0x8b0 [ibmvfc]
[c000000288d1bb70] [c0000000000d8c68] vio_bus_remove+0x68/0x100
[c000000288d1bbb0] [c0000000007da7c4] device_release_driver_internal+0x1f4/0x2d0
[c000000288d1bc00] [c0000000007da95c] driver_detach+0x7c/0x100
[c000000288d1bc40] [c0000000007d8af4] bus_remove_driver+0x84/0x140
[c000000288d1bcb0] [c0000000007db6ac] driver_unregister+0x4c/0xa0
[c000000288d1bd20] [c0000000000d6e7c] vio_unregister_driver+0x2c/0x50
[c000000288d1bd50] [d00000000290ba0c] cleanup_module+0x24/0x15e0 [ibmvfc]
[c000000288d1bd70] [c0000000001dadb0] sys_delete_module+0x220/0x2d0
[c000000288d1be30] [c00000000000b284] system_call+0x58/0x6c
Instruction dump:
e8410018 e87f0068 809f0078 e8bf0080 e8df0088 2fa30000 419e008c e9230200
2fa90000 419e0080 894d098a 794a07e0 <0b0a0000> e9290008 2fa90000 419e0028

This is tripped as a result of irqs being disabled during the call to
dma_free_coherent() by ibmvfc_free_event_pool(). At this point in the code path
we have quiesced the adapter and its overly paranoid anyways to be holding the
host lock.

Reported-by: Abdul Haleem <abdhalee@linux.vnet.ibm.com>
Signed-off-by: Tyrel Datwyler <tyreld@linux.vnet.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>