bcachefs: slab-use-after-free Read in bch2_sb_errors_from_cpu

Acquire fsck_error_counts_lock before accessing the critical section
protected by this lock.

syzbot has tested the proposed patch and the reproducer did not trigger
any issue.

Reported-by:  <syzbot+a2bc0e838efd7663f4d9@syzkaller.appspotmail.com>
Closes: https://syzkaller.appspot.com/bug?extid=a2bc0e838efd7663f4d9
Signed-off-by: Pei Li <peili.dev@gmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>