Revert "LU-8130 ptlrpc: convert conn_hash to rhashtable"

Revert "LU-8130 ptlrpc: convert conn_hash to rhashtable"

This reverts commit 7b3f9e5d6c509fabcec3cbd71e541a84987db2ff
due to crashes being seen on the MDS during client mount,
as described in LU-11624.

Change-Id: I3b39363ad1e41dee60f31466aa23d555ebe5135d
Reviewed-on: https://review.whamcloud.com/33595
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Shuichi Ihara <sihara@ddn.com>
Reviewed-by: James Simmons <uja.ornl@yahoo.com>
Reviewed-by: Patrick Farrell <paf@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>