LU-15986 ptlrpc: protect rq_repmsg in ptlrpc_req_drop_rs()

LU-15986 ptlrpc: protect rq_repmsg in ptlrpc_req_drop_rs()

There is a race condition that: on server side, one thread sent
reply message and is deleting the reply message, another is
searching for existing request and print some debug information
in _debug_req() if there is a duplicated request. They both operate on
req->rq_repmsg but it is not protected in ptlrpc_req_drop_rs().
So we protected it with req->rq_early_free_lock.

Signed-off-by: Lei Feng <flei@whamcloud.com>
Change-Id: Ied55427ee15c3ef84bdd2d579844eba398dbf010
Reviewed-on: https://review.whamcloud.com/47839
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>