udp: Unify buffers for tap and splice paths

We maintain a set of buffers for UDP packets to be forwarded via the tap
interface in udp[46]_l2_buf.  We then have a separate set of buffers for
packets to be "spliced" in udp_splice_buf[].  However, we only use one of
these at a time, so we can share the buffer space.

For the receiving splice packets we can not only re-use the data buffers
but also the udp[46]_l2_iov_sock and udp[46]_l2_mh_sock control structures.

For sending the splice packets we keep the same data buffers, but we need
specific control structures.  We create udp[46]_iov_splice - we can't
reuse udp_l2_iov_sock[] because we need to write iov_len as we're writing
spliced packets, but the tap path expects iov_len to remain the same (it
only uses it for receive).  Likewise we create udp[46]_mh_splice with the
mmsghdr structures for sending spliced packets.  As well as needing to
reference different iovs, these need to all reference udp_splice_namebuf
instead of individual msg_name fields for each slot.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

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