target: Improve system responsivity during I/O

While testing ib_srpt I noticed that the target system became
rather unresponsive during intensive I/O. The patch below made
my target system responsive again during I/O without decreasing
performance.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>