LU-9835 utils: Properly maintain remaining buffer length in jt_opt_threads

LU-9835 utils: Properly maintain remaining buffer length in jt_opt_threads

When we write to a buffer, we do need to reduce the size of it,
not just move the starting pointer.

Test-Parameters: trivial
Change-Id: I688e31d5434fb3040985b65a092d56ca30b69941
Signed-off-by: Oleg Drokin <oleg.drokin@intel.com>
Reviewed-on: https://review.whamcloud.com/28363
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Jian Yu <yujian@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Reviewed-by: Shaun Tancheff <stancheff@cray.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>