procfs: use an enum for possible hidepid values

Previously, the hidepid parameter was checked by comparing literal
integers 0, 1, 2.  Let's add a proper enum for this, to make the
checking more expressive:

        0 → HIDEPID_OFF
        1 → HIDEPID_NO_ACCESS
        2 → HIDEPID_INVISIBLE

This changes the internal labelling only, the userspace-facing interface
remains unmodified, and still works with literal integers 0, 1, 2.

No functional changes.

Link: http://lkml.kernel.org/r/1484572984-13388-2-git-send-email-djalal@gmail.com
Signed-off-by: Lafcadio Wluiki <wluikil@gmail.com>
Signed-off-by: Djalal Harouni <tixxdz@gmail.com>
Acked-by: Kees Cook <keescook@chromium.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

fs/proc/root.c

@@ -58,7 +58,8 @@ int proc_parse_options(char *options, struct pid_namespace *pid)
 		case Opt_hidepid:
 			if (match_int(&args[0], &option))
 				return 0;
-			if (option < 0 || option > 2) {
+			if (option < HIDEPID_OFF ||
+			    option > HIDEPID_INVISIBLE) {
 				pr_err("proc: hidepid value must be between 0 and 2.\n");
 				return 0;
 			}