overflow.h: Add arithmetic shift helper

Add shift_overflow() helper to assist driver authors in ensuring that
shift operations don't cause overflows or other odd conditions.

Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>
Signed-off-by: Leon Romanovsky <leonro@mellanox.com>
[kees: tweaked comments and commit log, dropped unneeded assignment]
Signed-off-by: Kees Cook <keescook@chromium.org>