kernel: Add helper macros for loop unrolling

This helps in easily initializing blocks of code (e.g. static calls and
keys).

UNROLL(N, MACRO, __VA_ARGS__) calls MACRO N times with the first
argument as the index of the iteration. This allows string pasting to
create unique tokens for variable names, function calls etc.

As an example:

	#include <linux/unroll.h>

	#define MACRO(N, a, b)            \
		int add_##N(int a, int b) \
		{                         \
			return a + b + N; \
		}

	UNROLL(2, MACRO, x, y)

expands to:

	int add_0(int x, int y)
	{
		return x + y + 0;
	}

	int add_1(int x, int y)
	{
		return x + y + 1;
	}

Tested-by: Guenter Roeck <linux@roeck-us.net>
Reviewed-by: Kees Cook <keescook@chromium.org>
Reviewed-by: Casey Schaufler <casey@schaufler-ca.com>
Reviewed-by: John Johansen <john.johansen@canonical.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Acked-by: Song Liu <song@kernel.org>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Acked-by: Casey Schaufler <casey@schaufler-ca.com>
Nacked-by: Tetsuo Handa <penguin-kernel@I-love.SAKURA.ne.jp>
Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Paul Moore <paul@paul-moore.com>