drop_monitor: Add error code to detect duplicate state changes

	Patch to add -EAGAIN error to dropwatch netlink message handling code.
-EAGAIN will be returned anytime userspace attempts to transition the state of
the drop monitor service to a state that its already in.  That allows user space
to detect this condition, so it doesn't wait for a success ACK that will never
arrive.  Tested successfully by me

Signed-off-by: Neil Horman <nhorman@tuxdriver.com>
Signed-off-by: David S. Miller <davem@davemloft.net>