Don't put error pointers in bch_fs, that's gross.

This fixes (?) the check in bch2_checksum_type_valid() - depending on
our error paths, or depending on what our error paths are doing it at
least makes the code saner.

Reported-by: syzbot+2e3cb81b5d1fe18a374b@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>