net: l3mdev: Fix kerneldoc warning

net/l3mdev/l3mdev.c:249: warning: Function parameter or member 'arg' not described in 'l3mdev_fib_rule_match'

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: David Ahern <dsahern@kernel.org>
Link: https://lore.kernel.org/r/20201028005059.930192-1-andrew@lunn.ch


Signed-off-by: Jakub Kicinski <kuba@kernel.org>