staging: lustre: ptlrpc: quiet warning for 2.1/2.5 connections

The Lustre 2.5.4 client will print a warning about connections with
2.1.3 servers, yet they are still supposed to be interoperable.
Increase the window of warning to be up to Lustre 2.5.50, since we
do not intend to allow interoperability between 2.1 and 2.6 systems.
This was from the time when major releases like Lustre 1.4 and 1.8
were many years apart would have no chance for interoperation.

Only print this message once per client, to avoid flooding the console
for connections to many servers or frequent network reconnections.
Server versions should all be nearly the same in any case.

Signed-off-by: Andreas Dilger <andreas.dilger@intel.com>
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-2528
Reviewed-on: http://review.whamcloud.com/7916


Reviewed-by: Bob Glossman <bob.glossman@intel.com>
Reviewed-by: Jian Yu <jian.yu@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>