Add MBTOM(), a macro that converts from an mbuf blocking disposition

flag (M_DONTWAIT / M_TRYWAIT) to a malloc(9) blocking disposition flag
(M_NOWAIT, M_WAITOK).  The semantic match isn't perfect, but for
scenarios where malloc data is used in the network stack, such as for
MAC labeling or for m_tags, we sometimes need to map from one to the
other to get the right blocking behavior.

Obtained from:	TrustedBSD Project
Sponsored by:	DARPA, Network Associates Laboratories

sys/sys/mbuf.h

@@ -272,6 +272,7 @@ struct mbstat {
 #define	M_DONTWAIT	0x4		/* don't conflict with M_NOWAIT */
 #define	M_TRYWAIT	0x8		/* or M_WAITOK */
 #define	M_WAIT		M_TRYWAIT	/* XXX: Deprecated. */
+#define	MBTOM(how)	((how) == M_TRYWAIT ? M_WAITOK : M_NOWAIT)
 
 #ifdef _KERNEL
 /*-