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.
Signed-off-by: Michael Ellerman <mpe@xxxxxxxxxxxxxx>