libmount: Avoid triggering autofs in lookup_umount_fs_by_statfs

libmount: Avoid triggering autofs in lookup_umount_fs_by_statfs

Currently, umount /foo results in a statfs("/foo") call, which triggers
autofs. This can create another mountpoint on /foo, which is then unmounted
later instead of the actual /foo at the time umount was called.

This is especially an issue for umount -R /bar, which just fails with
-EBUSY as the accidental mountpoint is never accounted for and so it tries
to umount /bar before /bar/someautofs.

Replace the direct statfs call with open(path, O_PATH) + fstatfs, which sees
the autofs mount directly, without triggering it.