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=1cecc37d87c4286e5543


Signed-off-by: Diogo Jahchan Koike <djahchankoike@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>