scsi: ibmvfc: Remove BUG_ON in the case of an empty event pool

In the Linux kernel, the following vulnerability has been resolved:

scsi: ibmvfc: Remove BUG_ON in the case of an empty event pool

In practice the driver should never send more commands than are allocated

to a queue's event pool. In the unlikely event that this happens, the code

asserts a BUG_ON, and in the case that the kernel is not configured to

crash on panic returns a junk event pointer from the empty event list

causing things to spiral from there. This BUG_ON is a historical artifact

of the ibmvfc driver first being upstreamed, and it is well known now that

the use of BUG_ON is bad practice except in the most unrecoverable

scenario. There is nothing about this scenario that prevents the driver

from recovering and carrying on.

Remove the BUG_ON in question from ibmvfc_get_event() and return a NULL

pointer in the case of an empty event pool. Update all call sites to

ibmvfc_get_event() to check for a NULL pointer and perfrom the appropriate

failure or recovery action.