bcachefs: fix incorrect pointer check in __bch2_subvolume_delete()

For some unknown reason, checks on struct bkey_s_c_snapshot and struct
bkey_s_c_snapshot_tree pointers are missing.

Therefore, I think it would be appropriate to fix the incorrect pointer checking
through this patch.

Fixes: 4bd06f07 ("bcachefs: Fixes for snapshot_tree.master_subvol")
Signed-off-by: Jeongjun Park <aha310510@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>