Since the addition of the slirp member to struct mbuf, the value of
SLIRP_MSIZE and the initialization of m_size have not been correct,
resulting in overrunning the end of the malloc'd buffer in some cases.

Signed-off-by: Bruce Rogers <brogers@novell.com>
Signed-off-by: Anthony Liguori <aliguori@us.ibm.com>