LU-18599 mdc: assign mod_close_req when RPC is ready

LU-18599 mdc: assign mod_close_req when RPC is ready

we shouldn't assign mod_close_req before the corresponding close RPC
is fully populated, otherwise mdc_replay_open() can be accessing RPC
with no rq_reqmsg set.

Signed-off-by: Alex Zhuravlev <bzzz@whamcloud.com>
Change-Id: I78f7ba2b18c7e9c431accce591cd4a921e25f4da
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/57587
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Feng Lei <flei@ddn.com>
Reviewed-by: Nikitas Angelinas <nikitas.angelinas@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>