LU-9514 ptlrpc: free reply buffer earlier for open RPC

LU-9514 ptlrpc: free reply buffer earlier for open RPC

It is unnecessary to keep the reply buffer for open RPC. Replay
related data has already been saved in the request buffer when
the RPC replied. If the open replay really happen, the replay
logic will alloc the reply buffer when needed.

On the other hand, the client always tries to alloc big enough
space to hold the open RPC reply since the client does not exactly
know how much data the server will reply to the client. So the reply
buffer may be quite larger than the real needed. Under such case,
keeping the large reply buffer for the open RPC will occupy a lot
of RAM as to OOM if the are too many open RPCs to be replayed.

This patch frees the reply buffer for the open RPC when only
the replay logic holds the last reference of the RPC.

Test-Parameters: envdefinitions=ONLY=51f testlist=sanity ostfilesystemtype=ldiskfs mdtfilesystemtype=ldiskfs
Test-Parameters: envdefinitions=ONLY=51f testlist=sanity ostfilesystemtype=zfs mdtfilesystemtype=zfs
Signed-off-by: Fan Yong <fan.yong@intel.com>
Change-Id: I1bea2456b8aa4e53a0b65143a48e617f836181a0
Reviewed-on: https://review.whamcloud.com/27208
Tested-by: Jenkins
Tested-by: Maloo <hpdd-maloo@intel.com>
Reviewed-by: Niu Yawei <yawei.niu@intel.com>
Reviewed-by: Jinshan Xiong <jinshan.xiong@intel.com>
Reviewed-by: Oleg Drokin <oleg.drokin@intel.com>