bcachefs: Ancient versions with bad bkey_formats are no longer supported

Syzbot found an assertion pop, by generating an ancient filesystem
version with an invalid bkey_format (with fields that can overflow) as
well as packed keys that aren't representable unpacked.

This breaks key comparisons in all sorts of painful ways.

Filesystems have been automatically rewriting nodes with such invalid
formats for years; we can safely drop support for them.

Reported-by:  <syzbot+8a0109511de9d4b61217@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>