LU-14939 lnet: Allow specifying a source NID for lnetctl ping

LU-14939 lnet: Allow specifying a source NID for lnetctl ping

Add a new --source option for lnetctl ping command. This allows the
user to specify a local NI from which to send the ping. This also
ensures that the specified destination NID is also used. Otherwise,
pings to multi-rail peers may end up going to a different peer NI
based on the multi-rail selection algorithm. The ability to specify
a source NI, and thus fix the destination NI, is a great help in
troubleshooting communication issues between multi-rail peers.

Add test to exercise lnetctl ping --source option.

Lustre-change: https://review.whamcloud.com/44727
Lustre-commit: 48ef9982c474a02c460293bce17c9e45f9829eab

HPE-bug-id: LUS-10296
Test-Parameters: trivial testlist=sanity-lnet
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: I454217b30a92414de537880f076a11a693b1f0b3
Reviewed-on: https://review.whamcloud.com/48297
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>