Commit 93821778 (udp: Fix rcv socket
locking) accidentally removed sk_drops increments for UDP IPV4
sockets.

This field can be used to detect incorrect sizing of socket receive
buffers.
Signed-off-by: NEric Dumazet <dada1@cosmosbay.com>
Signed-off-by: NDavid S. Miller <davem@davemloft.net>