LU-15279 ptlrpc: use a cached value

LU-15279 ptlrpc: use a cached value

Don't calculate a early reply size - use a cached,
as it don't changed after start

Signed-off-by: Alexey Lyashkov <alexey.lyashkov@hpe.com>
Change-Id: I3a6bd5013d0646b6165db52d6a7fb38b263756e6
Reviewed-on: https://review.whamcloud.com/45661
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andrew Perepechko <andrew.perepechko@hpe.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>