udp: introduce udp->udp_flags

According to syzbot, it is time to use proper atomic flags
for various UDP flags.

Add udp_flags field, and convert udp->corkflag to first
bit in it.

Signed-off-by: Eric Dumazet <edumazet@google.com>
Reviewed-by: Willem de Bruijn <willemb@google.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>