Currently code that wants to use set_memory_ro() etc, needs to include
asm/set_memory.h, which doesn't exist on all arches.  Some code knows it
only builds on arches which have the header, other code guards the
inclusion with an #ifdef, neither is ideal.

So create linux/set_memory.h.  This always exists, so users don't need
an #ifdef just to include the header.

When CONFIG_ARCH_HAS_SET_MEMORY=y it includes asm/set_memory.h,
otherwise it provides empty non-failing implementations.

Link: http://lkml.kernel.org/r/1498717781-29151-1-git-send-email-mpe@ellerman.id.auSigned-off-by: NMichael Ellerman <mpe@ellerman.id.au>
Acked-by: NDaniel Borkmann <daniel@iogearbox.net>
Acked-by: NKees Cook <keescook@chromium.org>
Acked-by: NLaura Abbott <labbott@redhat.com>
Signed-off-by: NAndrew Morton <akpm@linux-foundation.org>
Signed-off-by: NLinus Torvalds <torvalds@linux-foundation.org>