ip: limit use of gso_size to udp

The ipcm(6)_cookie field gso_size is set only in the udp path. The ip
layer copies this to cork only if sk_type is SOCK_DGRAM. This check
proved too permissive. Ping and l2tp sockets have the same type.

Limit to sockets of type SOCK_DGRAM and protocol IPPROTO_UDP to
exclude ping sockets.

v1 -> v2
- remove irrelevant whitespace changes

Fixes: bec1f6f6 ("udp: generate gso with UDP_SEGMENT")
Reported-by: Maciej Żenczykowski <maze@google.com>
Signed-off-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: David S. Miller <davem@davemloft.net>