linux-user: Fix length calculations in host_to_target_cmsg()

The handling of length calculations in host_to_target_cmsg()
was rather confused:
 * when checking for whether the target cmsg header fit in
   the remaining buffer, we were using the host struct size,
   not the target size
 * we were setting tgt_len to "target payload + header length"
   but then using it as if it were the target payload length alone
 * in various message type cases we weren't handling the possibility
   that host or target buffers were truncated

Fix these problems. The second one in particular is liable
to result in us overrunning the guest provided buffer,
since we will try to convert more data than is actually
present.

Fixes: https://bugs.launchpad.net/qemu/+bug/1701808
Reported-by: Bruno Haible  <bruno@clisp.org>
Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Message-Id: <1513345976-22958-2-git-send-email-peter.maydell@linaro.org>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>
