string.h: Introduce memset_after() for wiping trailing members/padding

string.h: Introduce memset_after() for wiping trailing members/padding

A common idiom in kernel code is to wipe the contents of a structure
after a given member. This is especially useful in places where there is
trailing padding. These open-coded cases are usually difficult to read
and very sensitive to struct layout changes. Introduce a new helper,
memset_after() that takes the target struct instance, the byte to write,
and the member name after which the zeroing should start.

Additionally adds memset_startat() for wiping trailing members _starting_
at a specific member instead of after a member, which is more readable
in certain circumstances, but doesn't include any preceding padding.

Cc: Steffen Klassert <steffen.klassert@secunet.com>
Cc: Herbert Xu <herbert@gondor.apana.org.au>
Cc: "David S. Miller" <davem@davemloft.net>
Cc: Jakub Kicinski <kuba@kernel.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Francis Laniel <laniel_francis@privacyrequired.com>
Cc: Vincenzo Frascino <vincenzo.frascino@arm.com>
Cc: Daniel Axtens <dja@axtens.net>
Cc: netdev@vger.kernel.org
Signed-off-by: Kees Cook <keescook@chromium.org>