l3mdev: Correct function names in the kerneldoc comments

Fix the following W=1 kernel build warning(s):

 net/l3mdev/l3mdev.c:111: warning: expecting prototype for l3mdev_master_ifindex(). Prototype was for l3mdev_master_ifindex_rcu() instead
 net/l3mdev/l3mdev.c:145: warning: expecting prototype for l3mdev_master_upper_ifindex_by_index(). Prototype was for l3mdev_master_upper_ifindex_by_index_rcu() instead

Reported-by: Hulk Robot <hulkci@huawei.com>
Signed-off-by: Xiongfeng Wang <wangxiongfeng2@huawei.com>
Reviewed-by: David Ahern <dsahern@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>