findmnt: error on --target /non-exist

findmnt: error on --target /non-exist

The original --target implementation (< v2.27) has been based on
stat(), so it was usable for valid paths only.

The new implementation is based on search in the mountinfo file, so it
works for arbitrary crazy path. This is not backwardly compatible and
if the path does not exist then it still returns at least root
directory mount entry.

This patch forces mnt_table_find_mountpoint() to check if the path is
valid before we search in the mountinfo file.

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