NFS: Fix rename of directory onto empty directory

 If someone tries to rename a directory onto an empty directory, we
 currently fail and return EBUSY.
 This patch ensures that we try the rename if both source and target
 are directories, and that we fail with a correct error of EISDIR if
 the source is not a directory.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>