staging/lustre/ptlrpc: track unreplied requests

The request xid was used to make sure the ost object timestamps
being updated by the out of order setattr/punch/write requests
properly. However, this mechanism is broken by the multiple rcvd
slot feature, where we deferred the xid assignment from request
packing to request sending.

This patch moved back the xid assignment to request packing, and
the manner of finding lowest unreplied xid is changed from scan
sending & delay list to scan a unreplied requests list.

This patch also skipped packing the known replied XID in connect
and disconnect request, so that we can make sure the known replied
XID is increased only on both server & client side.

Signed-off-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-on: http://review.whamcloud.com/16759
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5951


Reviewed-by: Gregoire Pichon <gregoire.pichon@bull.net>
Reviewed-by: Alex Zhuravlev <alexey.zhuravlev@intel.com>
Signed-off-by: Oleg Drokin <green@linuxhacker.ru>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>