NFSv3: Add emulation of the lookupp() operation

In order to use the open_by_filehandle() operations on NFSv3, we need
to be able to emulate lookupp() so that nfs_get_parent() can be used
to convert disconnected dentries into connected ones.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>