ipv4: Fix error return value in fib_convert_metrics()

BugLink: https://bugs.launchpad.net/bugs/1784409

The validation code modified by commit 5b5e7a0d ("net: metrics:
add proper netlink validation") is organised differently in older
kernel versions.  The fib_convert_metrics() function that is modified
in the backports to 4.4 and 4.9 needs to returns an error code, not a
success flag.
Signed-off-by: Ben Hutchings <ben.hutchings@codethink.co.uk>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Signed-off-by: Kleber Sacilotto de Souza <kleber.souza@canonical.com>
Signed-off-by: Khalid Elmously <khalid.elmously@canonical.com>

net/ipv4/fib_semantics.c

@@ -980,7 +980,7 @@ fib_convert_metrics(struct fib_info *fi, const struct fib_config *cfg)
 				return -EINVAL;
 		} else {
 			if (nla_len(nla) != sizeof(u32))
-				return false;
+				return -EINVAL;
 			val = nla_get_u32(nla);
 		}
 		if (type == RTAX_ADVMSS && val > 65535 - 40)