NFSv4.2: Fix a reference leak in nfs42_proc_layoutstats_generic

The caller of rpc_run_task also gets a reference that must be put.

Signed-off-by: Jeff Layton <jeff.layton@primarydata.com>
Cc: stable@vger.kernel.org # 4.2+
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>