bcachefs: Add extra padding in bkey_make_mut_noupdate()

This fixes a kasan splat in propagate_key_to_snapshot_leaves() -
varint_decode_fast() does reads (that it never uses) up to 7 bytes past
the end of the integer.
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/btree_update.h

@@ -220,7 +220,8 @@ static inline struct bkey_i *__bch2_bkey_make_mut_noupdate(struct btree_trans *t
 	if (type && k.k->type != type)
 		return ERR_PTR(-ENOENT);
 
-	mut = bch2_trans_kmalloc_nomemzero(trans, bytes);
+	/* extra padding for varint_decode_fast... */
+	mut = bch2_trans_kmalloc_nomemzero(trans, bytes + 8);
 	if (!IS_ERR(mut)) {
 		bkey_reassemble(mut, k);