mm/pkeys, powerpc, x86: Provide an empty vma_pkey() in linux/pkeys.h

Consolidate the pkey handling by providing a common empty definition
of vma_pkey() in pkeys.h when CONFIG_ARCH_HAS_PKEYS=n.

This also removes another entanglement of pkeys.h and
asm/mmu_context.h.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Reviewed-by: Ram Pai <linuxram@us.ibm.com>
Reviewed-by: Dave Hansen <dave.hansen@intel.com>