target: return CONFLICT only when SA key unmatched

PREEMPT (and PREEMPT AND ABORT) should return CONFLICT iff a specified
SERVICE ACTION RESERVATION KEY is specified and matches no existing
persistent reservation.

Without this patch, a PREEMPT will return CONFLICT if either all
reservations are held by the initiator (self preemption) or there is
nothing to preempt. According to the spec, both of these cases should
succeed.

Signed-off-by: Steven Allen <steven.allen@purestorage.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>