btrfs: add stack helpers for a few btrfs items

We don't have these defined in the kernel because we don't have any
users of these helpers.  However we do use them in btrfs-progs, so
define them to make keeping accessors.h in sync between progs and the
kernel easier.

Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Signed-off-by: David Sterba <dsterba@suse.com>