UMA mbuf allocator use after free detection

The attached patch uses the trash ctor/dtor routines from uma_dbg to help 
detect use after free conditions for mbufs, and mbuf clusters.  It doesn't 
seem to cause any unexpected problems with xl, ath, or wi, but it does 
cause issues with iwi.  That is good, because iwi has some problems that 
need to be resolved.

I'd appreciate it if people could apply the patch and see if it causes any 
panics or unexpected behavior on their systems.  If all mbuf usage is 
correct, there should be no visible effect.

This code is of course only active when you have INVARIANTS compiled in so 
that it does not slow down performance otherwise.

Thanks,

Mike "Silby" Silbersack