LU-10754 tests: Clear mdc locks before tests

LU-10754 tests: Clear mdc locks before tests

On ZFS testing, a sync stemming from a lock cancellation
from a previous test sometimes causes us to run longer than
the sleep times allowed for forked processes to be ready.

So, cancel the MDC lru locks first.  This will only incur a
sync if there is data to sync, but will wait for one if
necessary.

Test-Parameters: testlist=sanityn,sanityn,sanityn,sanityn

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I865de238aadd6da719066e6f22e2a36d1d3f368e
Reviewed-on: https://review.whamcloud.com/34848
Tested-by: Jenkins
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>