bcachefs: Don't rely on snapshot_tree.master_subvol for reattaching

Previously, fsck used the snapshot tree's master subvol for finding the
root inode number - but the master subvol might have been deleting, and
setting a new one should be a user operation; meaning we can't rely on
it existing.

Fortunately, for finding the root inode number in a tree of snapshots,
finding any associated subvolume works.

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