target: Make se_tmr_lock IRQ-safe

transport_lookup_tmr_lun() can be called from interrupt context and
therefore needs to use IRQ-safe spinlock functions.  Fix this up, and
to make the locking work, convert the other uses of se_tmr_lock to be
IRQ-disabling.

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