nfs41: Verify channel's attributes accordingly to RFC v2

 ca_maxoperations:

      For the backchannel, the server MUST
      NOT change the value the client offers.  For the fore channel,
      the server MAY change the requested value.

  ca_maxrequests:

       For the backchannel, the server MUST NOT change the
       value the client offers.  For the fore channel, the server MAY
       change the requested value.

Signed-off-by: Vitaliy Gusev <gusev.vitaliy@nexenta.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>