SUNRPC: Use kmemdup_nul() in rpc_parse_scope_id()

Using kmemdup_nul() is more efficient when the length is known.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>