bcachefs: return err ptr instead of null in read sb clean

syzbot reported a null-ptr-deref in bch2_fs_start. [0]

When a sb is marked clear but doesn't have a clean section
bch2_read_superblock_clean returns NULL which PTR_ERR_OR_ZERO
lets through, eventually leading to a null ptr dereference down
the line. Adjust read sb clean to return an ERR_PTR indicating the
invalid clean section.

[0] https://syzkaller.appspot.com/bug?extid=1cecc37d87c4286e5543

Reported-by: syzbot+1cecc37d87c4286e5543@syzkaller.appspotmail.com
Closes: https://syzkaller.appspot.com/bug?extid=1cecc37d87c4286e5543Signed-off-by: Diogo Jahchan Koike <djahchankoike@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>

fs/bcachefs/sb-clean.c

@@ -155,7 +155,7 @@ struct bch_sb_field_clean *bch2_read_superblock_clean(struct bch_fs *c)
 		SET_BCH_SB_CLEAN(c->disk_sb.sb, false);
 		c->sb.clean = false;
 		mutex_unlock(&c->sb_lock);
-		return NULL;
+		return ERR_PTR(-BCH_ERR_invalid_sb_clean);
 	}
 
 	clean = kmemdup(sb_clean, vstruct_bytes(&sb_clean->field),