LU-10876 lnet: peer deletion code may hide error

lnet_peer_ni_del_locked might return -EBUSY if the
NID to be deleted is a gateway.

Check for the return value of lnet_peer_ni_del_locked
in lnet_peer_del_nid.

Change-Id: I72f0d9355284bcc14b2827df82765b95a6d589f0
Test-Parameters: trivial
Signed-off-by: Sonia Sharma <sonia.sharma@intel.com>
Reviewed-on: https://review.whamcloud.com/31861
Tested-by: Jenkins
Reviewed-by: Dmitry Eremin <dmitry.eremin@intel.com>
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>