mm/pkey: Add PKEY_UNRESTRICTED macro

Memory protection keys (pkeys) uapi has two macros for pkeys restrictions:

 - PKEY_DISABLE_ACCESS 0x1
 - PKEY_DISABLE_WRITE  0x2

with implicit literal value of 0x0 that means "unrestricted". Code that
works with pkeys has to use this literal value when implying that a pkey
imposes no restrictions. This may reduce readability because 0 can be
written in various ways (e.g. 0x0 or 0) and also because 0 in the context
of pkeys can be mistaken for "no permissions" (akin PROT_NONE) while it
actually means "no restrictions". This is important because pkeys are
oftentimes used near mprotect() that uses PROT_ macros.

This patch adds PKEY_UNRESTRICTED macro defined as 0x0.

Signed-off-by: Yury Khrustalev <[email protected]>
Acked-by: Dave Hansen <[email protected]>
Reviewed-by: Kevin Brodsky <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Catalin Marinas <[email protected]>

include/uapi/asm-generic/mman-common.h

@@ -85,6 +85,7 @@
 /* compatibility flags */
 #define MAP_FILE	0
 
+#define PKEY_UNRESTRICTED	0x0
 #define PKEY_DISABLE_ACCESS	0x1
 #define PKEY_DISABLE_WRITE	0x2
 #define PKEY_ACCESS_MASK	(PKEY_DISABLE_ACCESS |\