bcachefs: Fix build errors with gcc 10

gcc 10 seems to complain about array bounds in situations where gcc 11
does not - curious.

This unfortunately requires adding some casts for now; we may
investigate getting rid of our __u64 _data[] VLA in a future patch so
that our start[0] members can be VLAs.

Reported-by: John Stoffel <john@stoffel.org>
Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>