NFSv4.1: add tracepoint to trunked nfs4_exchange_id calls

Add a tracepoint to track when the client sends EXCHANGE_ID to test
a new transport for session trunking.

nfs4_detect_session_trunking() tests for trunking and returns
EINVAL if trunking can't be done, add EINVAL mapping to
show_nfs4_status() in tracepoints.

Signed-off-by: Olga Kornievskaia <kolga@netapp.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>