sosendudp: Try to optimize out the additional mbuf alloc on output path

sosendudp: Try to optimize out the additional mbuf alloc on output path

This optimization leaves enough space at the beginning of the mbuf, so
later on M_PREPEND() probably will not allocate addition mbuf.

This probably will not benefit any data that will be fragmented, e.g. by
IPv4, so this optimization is only performed when the size of data and
max size of protocol+link headers fit into one mbuf cluster.

This optimization could be turned off by net.inet.udp.sosend_prepend,
which is on by default.

• [DBH] sys/kern/uipc_socket.c
• [DBH] sys/netinet/udp_usrreq.c