Implement VOP_VPTOCNP() for tmpfs.

For directories, node->tn_spec.tn_dir.tn_parent pointer to the parent
is used. For non-directories, the implementation is naive, all
directory nodes are scanned to find a dirent linking the specified
node. This can be significantly improved by maintaining tn_parent for
all nodes, later.

Tested by: pho (as part of larger patch)
Sponsored by: The FreeBSD Foundation
MFC after: 2 weeks