tests: (lsfd::mkfds-inotify) use findmnt(1) instead of stat(1) to get bdev numbers

tests: (lsfd::mkfds-inotify) use findmnt(1) instead of stat(1) to get bdev numbers

Close #2349

stat(2) on some filesystems including btrfs reports a block device
number (bdev number) with a offset. On the other hand, "inotify wd"
field of /proc/$pid/fdinfo/$fd for an inotify file reports bdev
numbers without offsets.

The offset caused the failure of lsfd::mkfds-inotify test case on
btrfs.  Unlike stat(2), findmnt can report bdev numbers without
offsets.

Signed-off-by: Masatake YAMATO <yamato@redhat.com>