bcachefs: bch2_snapshot_is_ancestor() now safe to call in early recovery

this fixes an assertion pop in
  bch2_check_snapshot_trees() ->
  check_snapshot_tree() ->
  bch2_snapshot_tree_master_subvol() ->
  bch2_snapshot_is_ancestor()

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