bcachefs: Inline fast path of check_pos_snapshot_overwritten()

This moves the slowpath of check_pos_snapshot_overwritten() to a
separate function, and inlines the fast path - helping performance on
btrees that don't use snapshot and for users that aren't using
snapshots.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_update_leaf.c

@@ -1113,7 +1113,7 @@ int __bch2_trans_commit(struct btree_trans *trans)
 	goto retry;
 }
 
-static int check_pos_snapshot_overwritten(struct btree_trans *trans,
+static noinline int __check_pos_snapshot_overwritten(struct btree_trans *trans,
 					  enum btree_id id,
 					  struct bpos pos)
 {
@@ -1122,12 +1122,6 @@ static int check_pos_snapshot_overwritten(struct btree_trans *trans,
 	struct bkey_s_c k;
 	int ret;
 
-	if (!btree_type_has_snapshots(id))
-		return 0;
-
-	if (!snapshot_t(c, pos.snapshot)->children[0])
-		return 0;
-
 	bch2_trans_iter_init(trans, &iter, id, pos,
 			     BTREE_ITER_NOT_EXTENTS|
 			     BTREE_ITER_ALL_SNAPSHOTS);
@@ -1153,6 +1147,18 @@ static int check_pos_snapshot_overwritten(struct btree_trans *trans,
 	return ret;
 }
 
+static inline int check_pos_snapshot_overwritten(struct btree_trans *trans,
+					  enum btree_id id,
+					  struct bpos pos)
+{
+	if (!btree_type_has_snapshots(id) ||
+	    pos.snapshot == U32_MAX ||
+	    !snapshot_t(trans->c, pos.snapshot)->children[0])
+		return 0;
+
+	return __check_pos_snapshot_overwritten(trans, id, pos);
+}
+
 static noinline int extent_front_merge(struct btree_trans *trans,
 				       struct btree_iter *iter,
 				       struct bkey_s_c k,