seccomp: swap hard-coded zeros to defined name

For clarity, if CONFIG_SECCOMP isn't defined, seccomp_mode() is returning
"disabled". This makes that more clear, along with another 0-use, and
results in no operational change.

Signed-off-by: Kees Cook <keescook@chromium.org>

include/linux/seccomp.h

@@ -78,7 +78,7 @@ static inline long prctl_set_seccomp(unsigned long arg2, char __user *arg3)
 
 static inline int seccomp_mode(struct seccomp *s)
 {
-	return 0;
+	return SECCOMP_MODE_DISABLED;
 }
 #endif /* CONFIG_SECCOMP */