staging: lustre: ptlrpc: convert conn_hash to rhashtable

Linux has a resizeable hashtable implementation in lib,
so we should use that instead of having one in libcfs.

This patch converts the ptlrpc conn_hash to use rhashtable.
In the process we gain lockless lookup.

As connections are never deleted until the hash table is destroyed,
there is no need to count the reference in the hash table.  There
is also no need to enable automatic_shrinking.

Signed-off-by: NeilBrown <neilb@suse.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>