staging/lustre/ptlrpc: avoid list scan in ptlrpcd_check

ptlrpcd_check() always scan all requests on ptlrpc_request_set
and try to finish completed requests, this is low efficiency.
Even worse, l_wait_event() always checks condition for twice
before sleeping and one more time after waking up, which means
it will call ptlrpcd_check() for three times in each loop.

This patch will move completed requests at the head of list
in ptlrpc_check_set(), with this change ptlrpcd_check doesn't
need to scan all requests anymore.

Signed-off-by: Liang Zhen <liang.zhen@intel.com>
Reviewed-on: http://review.whamcloud.com/11513
Intel-bug-id: https://jira.hpdd.intel.com/browse/LU-5548


Reviewed-by: Bobi Jam <bobijam@gmail.com>
Reviewed-by: Andreas Dilger <andreas.dilger@intel.com>
Reviewed-by: Johann Lombardi <johann.lombardi@intel.com>
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>