udp: make function udp_skb_dtor_locked static

Function udp_skb_dtor_locked does not need to be in global scope
so make it static to fix sparse warning:

net/ipv4/udp.c: warning: symbol 'udp_skb_dtor_locked' was not
declared. Should it be static?

Fixes: 6dfb4367 ("udp: keep the sk_receive_queue held when splicing")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Paolo Abeni <pabeni@redhat.com>
Signed-off-by: David S. Miller <davem@davemloft.net>