net: rules: Move l3mdev attribute validation to a helper

Move the check on FRA_L3MDEV attribute to helper to improve the
readability of fib_nl2rule. Update the extack messages to be
clear when the configuration option is disabled versus an invalid
value has been passed.

Signed-off-by: David Ahern <dsahern@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>