fs: btrfs: Crossport structure accessor into ctree.h

This brings all structure accessors from btrfs-progs/ctree.h, as in
kernel's ctree.h.

All these accessors handle the endian convert at runtime, and since all
of them are defined as static inline functions, those which aren't used
won't take space in resulting binary.

Signed-off-by: Qu Wenruo <wqu@suse.com>
Reviewed-by: Marek Behún <marek.behun@nic.cz>