LU-9971 lnet: use after free in lnet_discover_peer_locked()

LU-9971 lnet: use after free in lnet_discover_peer_locked()

When the lnet_net_lock is unlocked, the peer attached to an
lnet_peer_ni (found via lnet_peer_ni::lpni_peer_net->lpn_peer)
can change, and the old peer deallocated. If we are really
unlucky, then all the churn could give us a new, different,
peer at the same address in memory.

Change the reference counting on the lnet_peer lp so that it
is guaranteed to be alive when we relock the lnet_net_lock for
the cpt. When the reference count is dropped lp may go away if
it was unlinked, but the new peer is guaranteed to have a
different address, so we can still correctly determine whether
the peer changed and discovery should be redone.

Signed-off-by: Olaf Weber <olaf.weber@hpe.com>
Change-Id: Ia44dce20074b27ec0e77d7c1908c6a44ec73d326
Reviewed-on: https://review.whamcloud.com/28944
Reviewed-by: Amir Shehata <ashehata@whamcloud.com>
Tested-by: Jenkins
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>