HAMMER VFS - Fix two ip->vp inode/vnode races

HAMMER VFS - Fix two ip->vp inode/vnode races

* nrename and nremove attempt to dereference ip->vp.  Because ip->vp has
  no other references in these code paths the operation can race and
  either assert or panic on a null-pointer dereference.

  Fix by formally acquiring the vnode before trying to dereference it.

* All other cases are passed a referenced vnode or properly acquire it
  (e.g. nresolve), and did not have this issue.