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.

Signed-off-by: Olaf Faaland <faaland1@llnl.gov>
Change-Id: I1c67214f107ae2afe34d050470155807063bda51
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/58135
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>