IB/srp: Make writing into the "add_target" sysfs attribute interruptible

Avoid that stopping srp_daemon takes unusually long due to a cable
pull by making writing into the "add_target" sysfs attribute
interruptible.

Signed-off-by: Bart Van Assche <bvanassche@acm.org>
Signed-off-by: Roland Dreier <roland@purestorage.com>