bcachefs: Fix shift-by-64 in bformat_needs_redo()

Ancient versions of bcachefs produced packed formats that could
represent keys that our in memory format cannot represent;
bformat_needs_redo() has some tricky shifts to check for this sort of
overflow.

Reported-by:  <syzbot+594427aebfefeebe91c6@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>