If an invalid MTU value is set through rtnetlink return extra error
information instead of putting message in kernel log. For other cases
where there is no visible API, keep the error report in the log.

Example:
	# ip li set dev enp12s0 mtu 10000
	Error: mtu greater than device maximum.

	# ifconfig enp12s0 mtu 10000
	SIOCSIFMTU: Invalid argument
	# dmesg | tail -1
	[ 2047.795467] enp12s0: mtu greater than device maximum
Signed-off-by: Stephen Hemminger <stephen@networkplumber.org>
Signed-off-by: David S. Miller <davem@davemloft.net>