NFS: Refactor nfs_instantiate() for dentry referencing callers

Since commit b0c6108e

 ("nfs_instantiate(): prevent multiple aliases for
directory inode"), nfs_instantiate() may succeed without actually
instantiating the dentry that was passed in.  That can be problematic for
some callers in NFSv3, so this patch breaks things up so we can get the
actual dentry obtained.

Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>