udp: Remove redundant __udp_sysctl_init() call from udp_init().

__udp_sysctl_init() is called for init_net via udp_sysctl_ops.

While at it, we can rename __udp_sysctl_init() to udp_sysctl_init().

Fixes: 1e802951

 ("udp: Move the udp sysctl to namespace.")
Signed-off-by: Kuniyuki Iwashima <kuniyu@amazon.com>
Signed-off-by: David S. Miller <davem@davemloft.net>