bcachefs: Fix a UAF after write_super()

write_super() may reallocate the superblock buffer - but
bch_sb_field_ext was referencing it; don't use it after the write_super
call.

Reported-by:  <syzbot+8992fc10a192067b8d8a@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>