ixgbe/NFS m_defrag() instrumentation

[Garrett Wollman]

I instrumented calls to m_defrag() in ixgbe.  As expected, it gets
called *a lot* when NFS is running with the default read size of 64k.
A simple benchmark (single-threaded sequential read of a 128 GB file
which I didn't even run to completion) tells the tale:

$ sysctl dev.ix.0.mbuf_defrag_attempted
dev.ix.0.mbuf_defrag_attempted: 1737994

(There's already a similar counter for m_defrag() failures, which made
it easy to add this counter.  Unfortunately, there is no analogous
instrumentation in cxgbe so I couldn't do likewise for that NIC.)

-GAWollman