scsi: zfcp: Clarify & assert the stat_lock locking in zfcp_qdio_send()

scsi: zfcp: Clarify & assert the stat_lock locking in zfcp_qdio_send()

Explain why the plain spin_lock() suffices in current code, even when the
stat_lock is also used by zfcp_qdio_int_req() in tasklet context.

We could also promote the spin_lock() to a spin_lock_irqsave(), but that
would just obfuscate the locking even further.

Link: https://lore.kernel.org/r/b023b1472630f4bf9fce580577d7bb49de89ccbf.1603908167.git.bblock@linux.ibm.com
Reviewed-by: Benjamin Block <bblock@linux.ibm.com>
Reviewed-by: Steffen Maier <maier@linux.ibm.com>
Signed-off-by: Julian Wiedmann <jwi@linux.ibm.com>
Signed-off-by: Benjamin Block <bblock@linux.ibm.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>