bcachefs: Fix creating snapshot with implict source

When creating a snapshot without specifying the source subvolume, we use
the subvolume containing the new snapshot.

Previously, this worked if the directory containing the new snapshot was
the subvolume root - but we were using the incorrect helper, and got a
subvolume ID of 0 when the parent directory wasn't the root of the
subvolume, causing an emergency read-only.

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>