LU-9480 lnet: cleanup of lnet_peer_ni_addref/decref_locked()

LU-9480 lnet: cleanup of lnet_peer_ni_addref/decref_locked()

Address style issues in lnet_peer_ni_addref_locked() and
lnet_peer_ni_decref_locked(). In the latter routine, replace
a sequence of atomic_dec()/atomic_read() with atomic_dec_and_test().

Test-Parameters: trivial
Signed-off-by: Olaf Weber <olaf@sgi.com>
Change-Id: I9b7030ac9850b035f8bd80487a7b69b66b1d5858
Reviewed-on: https://review.whamcloud.com/25777
Reviewed-by: Olaf Weber <olaf.weber@hpe.com>
Reviewed-by: Amir Shehata <amir.shehata@intel.com>
Tested-by: Amir Shehata <amir.shehata@intel.com>