LU-16100 tests: fix sanity/51d divide-by-zero

LU-16100 tests: fix sanity/51d divide-by-zero

Fix dirstripe count when testing on non-DNE configs.

Test-Parameters: trivial
Fixes: cf35c54224b3 ("LU-14745 tests: ensure sanity/51d has enough objects")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I787df4cfda9e62673e5f89d2b899154f636777fe
Reviewed-on: https://review.whamcloud.com/48393
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: James Simmons <jsimmons@infradead.org>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Feng, Lei <flei@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>