NFS: Fix sysfs server name memory leak

Free the formatted server index string after it has been duplicated by
kobject_rename().

Fixes: 1c725118

 ("NFS: add superblock sysfs entries")
Reported-by: Alexander Aring <aahringo@redhat.com>
Signed-off-by: Benjamin Coddington <bcodding@redhat.com>
Tested-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>