LU-14139 ptlrpc: grow PtlRPC properly when prepare sub request

LU-14139 ptlrpc: grow PtlRPC properly when prepare sub request

In this patch, it prepares and grows PtlRPC reply buffer
properly for SUB batch request in @req_capsule_server_pack().

At the same time, it adds a limit of reply buffer size with
BUT_MAXREPSIZE = (1000 * 1024).

Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I4277974b3b0e9cd19fd0d18ae7c029cccaa9c558
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/43707
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Mikhail Pershin <mpershin@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>