NFSv4.1: don't do two EXCHANGE_IDs on mount

Since the addition of NFSv4 server trunking detection the mount context
calls nfs4_proc_exchange_id then schedules the state manager, which also
calls nfs4_proc_exchange_id. Setting the NFS4CLNT_LEASE_CONFIRM bit
makes the state manager skip the unneeded EXCHANGE_ID and continue on
with session creation.

Reported-by: Jorge Mora <mora@netapp.com>
Signed-off-by: Weston Andros Adamson <dros@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>