slirp: Correct size check in m_inc()

The data in an mbuf buffer is not necessarily at the start of the
allocated buffer. (For instance m_adj() allows data to be trimmed
from the start by just advancing the pointer and reducing the length.)
This means that the allocated buffer size (m->m_size) and the
amount of space from the m_data pointer to the end of the
buffer (M_ROOM(m)) are not necessarily the same.

Commit 864036e2 tried to change the m_inc() function from
taking the new allocated-buffer-size to taking the new room-size,
but forgot to change the initial "do we already have enough space"
check. This meant that if we were trying to extend a buffer which
had a leading gap between the buffer start and the data, we might
incorrectly decide it didn't need to be extended, and then
overrun the end of the buffer, causing memory corruption and
an eventual crash.

Change the "already big enough?" condition from checking the
argument against m->m_size to checking against M_ROOM().
This only makes a difference for the callsite in m_cat();
the other three callsites all start with a freshly allocated
mbuf from m_get(), which will have m->m_size == M_ROOM(m).

Fixes: 864036e2
Fixes: https://bugs.launchpad.net/qemu/+bug/1785670


Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Samuel Thibault <samuel.thibault@ens-lyon.org>
Message-id: 20180807114501.12370-1-peter.maydell@linaro.org
Tested-by: Dr. David Alan Gilbert <dgilbert@redhat.com>