nfs41: make sure nfs server return right ca_maxresponsesize_cached

According to rfc5661,

  ca_maxresponsesize_cached:

     Like ca_maxresponsesize, but the maximum size of a reply that
     will be stored in the reply cache (Section 2.10.6.1).  For each
     channel, the server MAY decrease this value, but MUST NOT
     increase it.

the latest kernel(2.6.38-rc8) may increase the value for ignoring
request's ca_maxresponsesize_cached value. We should not ignore it.

Signed-off-by: Mi Jinlong <mijinlong@cn.fujitsu.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>