bcachefs: Fix a memory splat

In __bch2_sb_field_resize, when a field's old a new size was 0, we were
doing an invalid write just past the end of the superblock.
Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/super-io.c

@@ -54,7 +54,9 @@ static struct bch_sb_field *__bch2_sb_field_resize(struct bch_sb_handle *sb,
 	BUG_ON(get_order(__vstruct_bytes(struct bch_sb, sb_u64s)) >
 	       sb->page_order);
 
-	if (!f) {
+	if (!f && !u64s) {
+		/* nothing to do: */
+	} else if (!f) {
 		f = vstruct_last(sb->sb);
 		memset(f, 0, sizeof(u64) * u64s);
 		f->u64s = cpu_to_le32(u64s);