bcachefs: vstruct_for_each() now declares loop iter

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

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>

bcachefs: Fix 32 bit build

vstruct_bytes() was returning a u64 - it should be a size_t, the corect
type for the size of anything that fits in memory.

Also replace a 64 bit divide with div_u64().

Signed-off-by: Kent Overstreet <kent.overstreet@gmail.com>

bcachefs: Initial commit

Initially forked from drivers/md/bcache, bcachefs is a new copy-on-write
filesystem with every feature you could possibly want.

Website: https://bcachefs.org

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