LU-13049 lnet: peer lookup handle shutdown

LU-13049 lnet: peer lookup handle shutdown

When LNet is shutting down, looking up peer_nis shouldn't assert
but return NULL. Callers handle NULL return

Test-Parameters: trivial
Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: Ia658f527719a71b2d0bed144ae03582eff54fcf9
Reviewed-on: https://review.whamcloud.com/36925
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Reviewed-by: Neil Brown <neilb@suse.de>
Reviewed-by: Oleg Drokin <green@whamcloud.com>