NFSv4: Fix double frees in nfs4_test_session_trunk()

rpc_clnt_add_xprt() expects the callback function to be synchronous, and
expects to release the transport and switch references itself.

Fixes: 04fa2c6b

 ("NFS pnfs data server multipath session trunking")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>