test: check copy_to/from_user boundary validation

test: check copy_to/from_user boundary validation

To help avoid an architecture failing to correctly check kernel/user
boundaries when handling copy_to_user, copy_from_user, put_user, or
get_user, perform some simple tests and fail to load if any of them behave
unexpectedly.

Specifically, this is to make sure there is a way to notice if things like
what was fixed in 8404663f81 ("ARM: 7527/1: uaccess: explicitly check
__user pointer when !CPU_USE_DOMAINS") ever regresses again, for any
architecture.

Additionally, adds new "user" selftest target, which loads this module.

Signed-off-by: Kees Cook <keescook@chromium.org>
Cc: Rusty Russell <rusty@rustcorp.com.au>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>