checkpatch.pl: add "prefer __packed" check

There's a __packed #define for __attribute__((packed)).  Add a checkpatch
to tell people about it.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Andy Whitcroft <apw@shadowen.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>