LU-16243 tests: Specify ping source in test_218

LU-16243 tests: Specify ping source in test_218

In sanity-lnet test_218 we want to drop all traffic from "nid1" to
itself. Use the --source argument to lnetctl ping command to ensure
that the ping is not sent from nid2 to nid1.

HPE-bug-id: LUS-11275
Test-Parameters: trivial
Signed-off-by: Chris Horn <chris.horn@hpe.com>
Change-Id: Ib82f182e1da5af303d4763d090d868196eb0ad70
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/48891
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Frank Sehr <fsehr@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>