syscall: silence scan-build dead-store in do_fchmodat_nofollow fallback

When neither AT_FDCWD nor AT_SYMLINK_NOFOLLOW is available, the function body is
a no-op warning that never reads mode or dfd, so the leading 'mode &= CHMOD_BITS'
became a dead store and dfd an unused parameter -- which the pinned clang-18
scan-build gate flags (deadcode.DeadStores).  Move the mask inside the
AT_SYMLINK_NOFOLLOW guard where mode is actually used, and mark dfd/mode used in
the fallback.  No behavior change on any platform that has the symlink-safe
primitive.

syscall.c

@@ -1485,8 +1485,8 @@ int do_chmod(const char *path, mode_t mode)
  * exists we skip with a warning rather than follow the leaf. */
 static int do_fchmodat_nofollow(int dfd, const char *name, mode_t mode)
 {
-	mode &= CHMOD_BITS;
 #if defined AT_FDCWD && defined AT_SYMLINK_NOFOLLOW
+	mode &= CHMOD_BITS;
 # if defined __linux__
 	{
 		STRUCT_STAT st;
@@ -1532,6 +1532,8 @@ static int do_fchmodat_nofollow(int dfd, const char *name, mode_t mode)
 	return fchmodat(dfd, name, mode, AT_SYMLINK_NOFOLLOW);
 # endif
 #else
+	(void)dfd;
+	(void)mode;
 	/* No symlink-safe chmod primitive here: skip rather than follow the leaf. */
 	rprintf(FWARNING, "do_chmod: no symlink-safe chmod for \"%s\"; mode not set\n", name);
 	return 1;