scsi: target: iscsi: Avoid in_interrupt() usage in iscsit_check_session_usage_count()

iscsit_check_session_usage_count() uses in_interrupt() to find out if it is
safe to invoke wait_for_completion().

The usage of in_interrupt() in drivers is phased out and Linus clearly
requested that code which changes behaviour depending on context should
either be separated or the context be conveyed in an argument passed by the
caller, which usually knows the context.

There is only one caller of iscsit_check_session_usage_count() which
already has an argument indicating if it is safe to sleep.

Extend iscsit_check_session_usage_count() by an argument indicating if it
may sleep.

Link: https://lore.kernel.org/r/20201220203638.43615-3-bigeasy@linutronix.de


Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>