LU-18738 utils: avoid statx() of root of mounted FS

LU-18738 utils: avoid statx() of root of mounted FS

When looking for a specific mounted lustre file system by path, avoid
the stat() or statx() call on lustre file systems whose mountpoints do
not match the given path.

This avoids hangs if the client is disconnected from MDT0 of other
mounted file systems, but the desired file system is reachable.

Lustre-change: https://review.whamcloud.com/58135
Lustre-commit: 2da8542e7069af71566a5d36d53fdc840a63228a

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: I1c67214f107ae2afe34d050470155807063bda51
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58321
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Gian-Carlo DeFazio <defazio1@llnl.gov>
Reviewed-by: Oleg Drokin <green@whamcloud.com>