illumos gate

The logic for generating and maintaining the cached v_path value on vnodes could stand to be improved. If vnodes were purely ephemeral, then freshly calculating v_path at the time of lookup() would result in correct values (at a performance cost). When they persist, either as referenced by other structures (such as open files, process cwd, dnlc entries, etc), the opportunity for the v_path to become stale arises. This is exacerbated by the current behavior that, when v_path is found to be invalid (during a vnodetopath operation) will strive to recalculate it, but not preserve the result. The overall situation leads to both performance and correctness (due to lack of results) problems relating to v_path.

This has been addressed in SmartOS through a series of changes. Firstly, to do proper invalidation of v_path when it's found to be stale:
- OS-3891 stale v_path slows vfs lookups

OS-3891 revealed that some logic made assumptions about v_path never transitioning from non-NULL to NULL. It was addressed here:
- OS-4317 v_path accesses can race

While the pathological stale v_path behavior had been addressed, there are still cases where the absence of valid v_path information was causing problems. The largest patch in this series addressed it by performing v_path checking and updates during vnode lookups/updates, when it is most convenient:
- OS-5167 cached v_path should be kept fresh

Two smaller updates are included too, to prevent erroneous behavior introduced by the prior changes:
- OS-5846 procfs should follow VFS rules
- OS-6134 vn_reinit balks on zeroed vnodes