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>
diff --git a/include/linux/pkeys.h b/include/linux/pkeys.h
index ed06e1a..aad5466 100644
--- a/include/linux/pkeys.h
+++ b/include/linux/pkeys.h
@@ -13,6 +13,11 @@
 #define PKEY_DEDICATED_EXECUTE_ONLY 0
 #define ARCH_VM_PKEY_FLAGS 0
 
+static inline int vma_pkey(struct vm_area_struct *vma)
+{
+	return 0;
+}
+
 static inline bool mm_pkey_is_allocated(struct mm_struct *mm, int pkey)
 {
 	return (pkey == 0);