LU-12739 lnet: Don't queue msg when discovery has completed

In lnet_initiate_peer_discovery(), it is possible for the peer object
to change after the call to lnet_discover_peer_locked(), and it is
also possible for the peer to complete discovery between the first
call to lnet_peer_is_uptodate() and our placing the lnet_msg onto
the peer's lp_dc_pendq. After the call to lnet_discover_peer_locked()
check whether the, potentially new, peer object is up to date while
holding the lp_lock. If the peer is up to date, then we needn't
queue the message. Otherwise, we continue to hold the lock to place
the message on the peer's lp_dc_pendq.

Lustre-change: https://review.whamcloud.com/36139
Lustre-commit: 4ef62976448d6821df9aab3e720fd8d9d0bdefce

Test-Parameters: trivial testlist=sanity-lnet
Cray-bug-id: LUS-7596
Signed-off-by: Chris Horn <hornc@cray.com>
Change-Id: Ib3da7447588479bb35afcc3fe176b9120d915a89