scsi: target: core: Fix warning on realtime kernels

On realtime kernels, spin_lock_irq*(spinlock_t) do not disable the
interrupts, a call to irqs_disabled() will return false thus firing a
warning in __transport_wait_for_tasks().

Remove the warning and also replace assert_spin_locked() with
lockdep_assert_held()

Link: https://lore.kernel.org/r/20210531121326.3649-1-mlombard@redhat.com
Reviewed-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Maurizio Lombardi <mlombard@redhat.com>
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>