LU-16465 tests: update sanity test 806 to use save/restore_opencache

LU-16465 tests: update sanity test 806 to use save/restore_opencache

There are existing primitives to do this, so no need to opencode them

Fixes: dfb08bbf77 ("LU-16465 llite: fix LSOM blocks for ftruncate and close")
Test-Parameters: trivial testlist=sanity env=ONLY=806
Change-Id: Ibc0d34050999bfda8f56384752ede09cacc4df91
Signed-off-by: Oleg Drokin <green@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/c/fs/lustre-release/+/50606
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Etienne AUJAMES <eaujames@ddn.com>