NFSv4.1: Don't clobber the seqid if exchange_id returns a confirmed clientid

If the EXCHGID4_FLAG_CONFIRMED_R flag is set, the client is in theory
supposed to already know the correct value of the seqid, in which case
RFC5661 states that it should ignore the value returned.

Also ensure that if the sanity check in nfs4_check_cl_exchange_flags
fails, then we must not change the nfs_client fields.

Finally, clean up the code: we don't need to retest the value of
'status' unless it can change.

Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>