It's currently nearly impossible to trace what process places a hold on
a vnode, as the only ways holds are place is via the `VN_HOLD()` and
`VN_HOLD_CALLER()` macros, which inline the bumping of `v_count`. Adding
static DTrace probes to these macros would enable tracing of where
specific vnode references come from.
For completeness and symmetry, a similar static probe should be added to
`vn_rele()` and `vn_rele_dnlc()`.