bcachefs: bch2_mark_snapshot() now called like other triggers

This fixes a bug where bch2_mark_snapshot() wasn't called for existing
snapshot nodes being updated when child nodes were added.

This led to the data update path thinking the key being updated was for
a snapshot that didn't have children, causing it to fail to insert
whiteouts when splitting existing extents.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/subvolume.c

@@ -512,8 +512,7 @@ int bch2_snapshot_node_create(struct btree_trans *trans, u32 parent,
 		n->v.pad	= 0;
 		SET_BCH_SNAPSHOT_SUBVOL(&n->v, true);
 
-		ret   = bch2_trans_update(trans, &iter, &n->k_i, 0) ?:
-			bch2_mark_snapshot(trans, bkey_s_c_null, bkey_i_to_s_c(&n->k_i), 0);
+		ret   = bch2_trans_update(trans, &iter, &n->k_i, 0);
 		if (ret)
 			goto err;
 

fs/bcachefs/subvolume.h

@@ -8,15 +8,15 @@
 void bch2_snapshot_to_text(struct printbuf *, struct bch_fs *, struct bkey_s_c);
 int bch2_snapshot_invalid(const struct bch_fs *, struct bkey_s_c,
 			  unsigned, struct printbuf *);
+int bch2_mark_snapshot(struct btree_trans *, struct bkey_s_c,
+		       struct bkey_s_c, unsigned);
 
 #define bch2_bkey_ops_snapshot ((struct bkey_ops) {		\
 	.key_invalid	= bch2_snapshot_invalid,		\
 	.val_to_text	= bch2_snapshot_to_text,		\
+	.atomic_trigger	= bch2_mark_snapshot,			\
 })
 
-int bch2_mark_snapshot(struct btree_trans *, struct bkey_s_c,
-		       struct bkey_s_c, unsigned);
-
 static inline struct snapshot_t *snapshot_t(struct bch_fs *c, u32 id)
 {
 	return genradix_ptr(&c->snapshots, U32_MAX - id);