binfmt_flat: add endianess annotations

Most binfmt_flat on-disk fields are big endian.  Use the proper __be32
type where applicable.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Tested-by: Vladimir Murzin <vladimir.murzin@arm.com>
Reviewed-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Greg Ungerer <gerg@linux-m68k.org>