iscsi-target: Refactor iscsit_get_np sockaddr matching into iscsit_check_np_match

This patch refactors the sockaddr matching logic in iscsit_get_np() into
a seperate iscsit_check_np_match() that can be used by external code.

Tested-by: Andy Grover <agrover@redhat.com>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>