vfs: VOP_REMOVE: move vnode unlocking and ref dropping to FS-indep part

vfs: VOP_REMOVE: move vnode unlocking and ref dropping to FS-indep part

while here, ensure all vop_remove field are set, and always call the function.

the change is very conservative: it only adds vnode ref drop/unlock where it was
absent because it should be unreachable (and if it wasn't, it should fix
things).

ok miod@