vn_fullpath.9: document the name cache fallback's locking

[seuros]

Summary

The page says vp has no need to be locked, which reads as permission to
pass an exclusively locked vnode. A name cache miss then takes LK_SHARED
on that same vnode in the fallback path.

[mjg]

It is not legal to hold any vnode locks across a call to vn_fullpath. In particular having the vp locked threatens a deadlock against normal path lookup, as the reconstruction grabs one lock at a time the opposite way. So you can have the child locked and stall waiting for the parent, while the forward lookup just locked the parent and now wants the child.

[seuros]

reword