bcachefs: Allow for unknown key types in backpointers fsck

We can't assume that btrees only contain keys of a given type - even if
they only have a single key type listed in the allowed key types for
that btree; this is a forwards compatibility issue.

Reported-by:  <syzbot+a27c3aaa3640dd3e1dfb@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>