bcachefs: Fix assert in bch2_alloc_v4_invalid()

Reported-by: syzbot+10827fa6b176e1acf1d0@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/alloc_background.c

@@ -244,10 +244,10 @@ int bch2_alloc_v4_invalid(struct bch_fs *c, struct bkey_s_c k,
 	struct bkey_s_c_alloc_v4 a = bkey_s_c_to_alloc_v4(k);
 	int ret = 0;
 
-	bkey_fsck_err_on(alloc_v4_u64s(a.v) > bkey_val_u64s(k.k), c, err,
+	bkey_fsck_err_on(alloc_v4_u64s_noerror(a.v) > bkey_val_u64s(k.k), c, err,
 			 alloc_v4_val_size_bad,
 			 "bad val size (%u > %zu)",
-			 alloc_v4_u64s(a.v), bkey_val_u64s(k.k));
+			 alloc_v4_u64s_noerror(a.v), bkey_val_u64s(k.k));
 
 	bkey_fsck_err_on(!BCH_ALLOC_V4_BACKPOINTERS_START(a.v) &&
 			 BCH_ALLOC_V4_NR_BACKPOINTERS(a.v), c, err,

fs/bcachefs/alloc_background.h

@@ -126,13 +126,17 @@ static inline struct bpos alloc_freespace_pos(struct bpos pos, struct bch_alloc_
 	return pos;
 }
 
-static inline unsigned alloc_v4_u64s(const struct bch_alloc_v4 *a)
+static inline unsigned alloc_v4_u64s_noerror(const struct bch_alloc_v4 *a)
 {
-	unsigned ret = (BCH_ALLOC_V4_BACKPOINTERS_START(a) ?:
+	return (BCH_ALLOC_V4_BACKPOINTERS_START(a) ?:
 			BCH_ALLOC_V4_U64s_V0) +
 		BCH_ALLOC_V4_NR_BACKPOINTERS(a) *
 		(sizeof(struct bch_backpointer) / sizeof(u64));
+}
 
+static inline unsigned alloc_v4_u64s(const struct bch_alloc_v4 *a)
+{
+	unsigned ret = alloc_v4_u64s_noerror(a);
 	BUG_ON(ret > U8_MAX - BKEY_U64s);
 	return ret;
 }