NFS: Create a single nfs_get_root()

This patch splits out the NFS v4 specific functionality of
nfs4_get_root() into its own rpc_op called by the generic client, and
leaves nfs4_proc_get_rootfh() as its own stand alone function.  This
also allows me to change nfs4_remote_mount(), nfs4_xdev_mount() and
nfs4_remote_referral_mount() to use the generic client's nfs_get_root()
function.  Later patches in this series will collapse these functions
into one common function, so using the same get_root() function
everywhere simplifies future changes.

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