Add SDT probes virtqueue::enqueue_segments:{entry,return}

[stevek]

Summary

This allows one to be able to map the sglist entries passed into the
vq_ring_enqueue_segments() function to the segment indexes used in
the virtqueue.

This function normally gets inlined and may not available via FBT
probes.

Test Plan

Tested using dtrace on vtblk devices with the following script:
https://people.freebsd.org/~stevek/dtrace/vtblk.sh