LU-18160 lnet: ensure lnetctl ping completes in a finite time

LU-18160 lnet: ensure lnetctl ping completes in a finite time

Make the wait_for_completion_timeout() in lnet_ping() interruptible,
so users can halt lnetctl. Remove the second wait_for_completion()
after LNetMDUnlink(). Per the comments in lib-md.c, active messages
associated with the MD may get aborted upon unlinking. So just give
up on the ping.

Clean up some nearby code to make the logic more clear.

Signed-off-by: Timothy Day <timday@amazon.com>
Change-Id: Ia49c9b1f152f411ebabc5e1c00ff3f03a99f3596
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56110
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Oleg Drokin <green@whamcloud.com>
Reviewed-by: Serguei Smirnov <ssmirnov@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>