findmnt: don't rely on st_dev for --target

findmnt: don't rely on st_dev for --target

The overlay filesystem does not provide usable st_dev (in traditional
UNIX way). It's necessary to search in /proc/self/mountinfo to detect
which path element is mountpoint.

$ findmnt --target /mnt/merged/dir-a/foo
TARGET      SOURCE  FSTYPE  OPTIONS
/mnt/merged overlay overlay rw,relatime,lowerdir=/mnt/low,upperdir=/mnt/high/data,workdir=/mnt/high/work

Signed-off-by: Karel Zak <kzak@redhat.com>