Fix another shift-by-64 by factoring out a common helper for
bch2_bkey_format_invalid() and bformat_needs_redo() (where it was
already fixed).

Reported-by: syzbot+9833a1d29d4a44361e2c@syzkaller.appspotmail.com
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>