clone3: add build-time CLONE_ARGS_SIZE_VER* validity checks

CLONE_ARGS_SIZE_VER* macros are defined explicitly and not via
the offsets of the relevant struct clone_args fields, which makes
it rather error-prone, so it probably makes sense to add some
compile-time checks for them (including the one that breaks
on struct clone_args extension as a reminder to add a relevant
size macro and a similar check).  Function copy_clone_args_from_user
seems to be a good place for such checks.

Signed-off-by: Eugene Syromiatnikov <esyr@redhat.com>
Acked-by: Christian Brauner <christian.brauner@ubuntu.com>
Link: https://lore.kernel.org/r/20200412202658.GA31499@asgard.redhat.com
Signed-off-by: Christian Brauner <christian.brauner@ubuntu.com>