bcachefs: Array bounds fixes

bcachefs: Array bounds fixes

It's no longer legal to use a zero size array as a flexible array
member - this causes UBSAN to complain.

This patch switches our zero size arrays to normal flexible array
members when possible, and inserts casts in other places (e.g. where we
use the zero size array as a marker partway through an array).

Signed-off-by: Kent Overstreet <kent.overstreet@linux.dev>