add zfs_vptocnp with special handling for snapshots under .zfs

The logic is similar to that already present in zfs_dirlook() to handle
a dot-dot lookup on a root vnode of a snapshot mounted under
.zfs/snapshot/.
illumos does not have an equivalent of vop_vptocnp, so there only the
lookup had to be patched up.

MFC after: 4 weeks