target: Make se_dev_check_online() locking IRQ-safe

se_dev_check_online() is called from transport_lookup_cmd_lun(), which
as discussed before may be called from interrupt context.  So it needs
to use spin_lock_irqsave() instead of spin_lock_irq() to avoid
enabling interrupts at the wrong time.

Signed-off-by: Roland Dreier <roland@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>