nfs: don't bother setting/restoring export_path around do_nfs_root_mount()

nothing in it will be looking at that thing anyway

Reviewed-by: David Howells <dhowells@redhat.com>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>