LU-12385 lnet: update opa defaults

LU-12385 lnet: update opa defaults

Testing reveals no significant performance improvements
when using peer_credits > 32. Adjusted the default
peer_credits, peer_credits_hiw and concurrent_sends
to take that into account.

This has the advantage of avoiding an issue observed
on multiple opa sites where the qp can not be created because
of large initial queue_depth. The queue depth is then
reduced gradually until the qp creation succeeds.

Lustre-change: https://review.whamcloud.com/36072
Lustre-commit: 7f199dbf0261b89afe0dc8185db4403ae0efdefa

Signed-off-by: Amir Shehata <ashehata@whamcloud.com>
Change-Id: I6036ec1da7063e30b567446e5db89040f21bc701
Reviewed-by: Chris Horn <hornc@cray.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/36252
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>