net: sockets: tls: prevent sending fragmented datagrams with sendmsg()

Fragmented data passed to sendmsg() should be sent as a single datagram in
case of datagram sockets (i.e. DTLS connection). Right now that is not
happening now, as each fragment is sent separately, which works fine only
for stream sockets.

There is no mbedTLS API for 'gather' write at this moment. This means that
implementing sendmsg() would require allocating contiguous memory area at
Zephyr TLS socket level and copying all data fragments before passing to
mbedTLS library. While this might be a good option for future, let's just
check if data passed to sendmsg() API consists of a single memory region
and can be sent using single send request. Return EMSGSIZE error if there
are more then one data fragments.

Signed-off-by: Marcin Niestroj <m.niestroj@emb.dev>