path-util: fix breakage in path_is_mount_point

path-util: fix breakage in path_is_mount_point

This fixes 2 problems introduced by 6feeeab0bc:

1) If name_to_handle_at returns ENOSYS for the child, we'll wrongly
return -ENOSYS when it returns the same for the parent. Immediately
jump to the fallback logic when we get ENOSYS.

2) If name_to_handle_at returns EOPNOTSUPP for the child but suceeds
for the parent, we'll be comparing an uninitialized value (mount_id) to
an initialized value (mount_id_parent). Initialize the mount_id
variables to invalid mount_ids to avoid this.