slirp: Fix packet expiration

The two new variables "arp_requested" and "expiration_date" in the mbuf
structure have been added after the variable-sized "m_dat_" array. The
variables have to be added before the m_dat_ array instead.
Without this patch, the expiration_date gets clobbered by code that
accesses the m_dat_ array.
I experienced this problem with the code in slirp/tftp.c: The
tftp_send_data() function created a new packet with the m_get()
function (which fills-in a default expiration_date value). Then the
TFTP code cleared the data section of the packet, which accidentially
also cleared the expiration_date. This zeroed expiration_date then
finally causes the packet to be discarded during if_start(), so that
TFTP packets were not transmitted anymore.

[Jan: added comment as suggested by Fabien ]

CC: Fabien Chouteau <chouteau@adacore.com>
Signed-off-by: Thomas Huth <thuth@linux.vnet.ibm.com>
Signed-off-by: Jan Kiszka <jan.kiszka@siemens.com>

slirp/mbuf.h

@@ -82,12 +82,13 @@ struct m_hdr {
 struct mbuf {
 	struct	m_hdr m_hdr;
 	Slirp *slirp;
+	bool	arp_requested;
+	uint64_t expiration_date;
+	/* start of dynamic buffer area, must be last element */
 	union M_dat {
 		char	m_dat_[1]; /* ANSI don't like 0 sized arrays */
 		char	*m_ext_;
 	} M_dat;
-    bool     arp_requested;
-    uint64_t expiration_date;
 };
 
 #define m_next		m_hdr.mh_next