scsi: lpfc: Fix kasan slab-out-of-bounds error in lpfc_unreg_login

scsi: lpfc: Fix kasan slab-out-of-bounds error in lpfc_unreg_login

The following kasan bug was called out:

 BUG: KASAN: slab-out-of-bounds in lpfc_unreg_login+0x7c/0xc0 [lpfc]
 Read of size 2 at addr ffff889fc7c50a22 by task lpfc_worker_3/6676
 ...
 Call Trace:
 dump_stack+0x96/0xe0
 ? lpfc_unreg_login+0x7c/0xc0 [lpfc]
 print_address_description.constprop.6+0x1b/0x220
 ? lpfc_unreg_login+0x7c/0xc0 [lpfc]
 ? lpfc_unreg_login+0x7c/0xc0 [lpfc]
 __kasan_report.cold.9+0x37/0x7c
 ? lpfc_unreg_login+0x7c/0xc0 [lpfc]
 kasan_report+0xe/0x20
 lpfc_unreg_login+0x7c/0xc0 [lpfc]
 lpfc_sli_def_mbox_cmpl+0x334/0x430 [lpfc]
 ...

When processing the completion of a "Reg Rpi" login mailbox command in
lpfc_sli_def_mbox_cmpl, a call may be made to lpfc_unreg_login. The vpi is
extracted from the completing mailbox context and passed as an input for
the next. However, the vpi stored in the mailbox command context is an
absolute vpi, which for SLI4 represents both base + offset.  When used with
a non-zero base component, (function id > 0) this results in an
out-of-range access beyond the allocated phba->vpi_ids array.

Fix by subtracting the function's base value to get an accurate vpi number.

Link: https://lore.kernel.org/r/20200322181304.37655-2-jsmart2021@gmail.com
Signed-off-by: James Smart <jsmart2021@gmail.com>
Signed-off-by: Dick Kennedy <dick.kennedy@broadcom.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>