LU-16907 ptlrpc: correct the reply buffer size for batch RPC

LU-16907 ptlrpc: correct the reply buffer size for batch RPC

The calculation for growing reply buffer size for a batch RPC is
incorrect and it adds the SUB request size wrongly.
This may result in the following panic:
"Max IOV exceeded: 257 should be < 256"
Fix it accordingly.

Fixes: 5a2dfd36f9c ("LU-14139 ptlrpc: grow PtlRPC properly when prepare sub request")
Test-Parameters: testlist=sanity env=ONLY=123f,ONLY_REPEAT=10
Signed-off-by: Qian Yingjin <qian@ddn.com>
Change-Id: I3c5151a485cac7f3fb9384cd9fb022143ca3389d
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/56645
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Timothy Day <timday@amazon.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>