LU-15929 lnet: Correct net selection for router ping

LU-15929 lnet: Correct net selection for router ping

lnet_find_best_ni_on_local_net() contains logic for restricting
the NI selection to a net specified by lnet_peer::lp_disc_net_id. The
purpose of this is to ensure that LNet peers ping every interface on
a router at a regular interval as part of the LNet router health
feature. However, this logic is flawed because lnet_msg_discovery()
is used to determine whether the message being sent is a discovery
message, but that function actually determines whether a given message
can _trigger_ discovery.

Introduce a new function, lnet_msg_is_ping(), which determines whether
a given lnet_msg is a GET on the LNET_RESERVED_PORTAL.
Modify lnet_find_best_ni_on_local_net() to restrict NI selection to
lp_disc_net_id iff:
1. lp_disc_net_id is non-zero
2. The peer has the LNET_PEER_RTR_DISCOVERY flag set.
3. lnet_msg_is_ping() returns true

Test-Parameters: trivial testlist=sanity-lnet
HPE-bug-id: LUS-11017
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: I3dbdfd5c44b6167d24b7b6e0b1097db0b3c5cb76
Reviewed-on: https://review.whamcloud.com/47527
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Cyril Bordage <cbordage@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>