EX-7601 ofd: rename 'local' in thread_big_cache

EX-7601 ofd: rename 'local' in thread_big_cache

It's not a big deal since it's only used a few times, but
let's give this variable a descriptive name.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ide136cd42e885d59f1a2e4ce22a2e7449faca3f9
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52874
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>