udplite: fix various data-races

udp->pcflag, udp->pcslen and udp->pcrlen reads/writes are racy.

Move udp->pcflag to udp->udp_flags for atomicity,
and add READ_ONCE()/WRITE_ONCE() annotations for pcslen and pcrlen.

Fixes: ba4e58ec ("[NET]: Supporting UDP-Lite (RFC 3828) in Linux")
Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>