kernel/power/snapshot.c: use linux/set_memory.h

kernel/power/snapshot.c: use linux/set_memory.h

This header always exists, so doesn't require an ifdef around its
inclusion.  When CONFIG_ARCH_HAS_SET_MEMORY=y it includes the asm
header, otherwise it provides empty versions of the set_memory_xx()
routines.

Link: http://lkml.kernel.org/r/1498717781-29151-2-git-send-email-mpe@ellerman.id.au
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Acked-by: Kees Cook <keescook@chromium.org>
Acked-by: Laura Abbott <labbott@redhat.com>
Cc: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>