bcachefs: Fix compat issue with old alloc_v4 keys

we allow new fields to be added to existing key types, and new versions
should treat them as being zeroed; this was not handled in
alloc_v4_validate.

Reported-by:  <syzbot+3b2968fa4953885dd66a@syzkaller.appspotmail.com>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>