LU-13321 tests: force even DNE file distribution

LU-13321 tests: force even DNE file distribution

Force even file creation across DNE-striped directories in sanity
test_160f to avoid random test failures (about 15%) when no files
are created on a specific MDT.

Fixes: 0a1cf8da8069 ("LU-11025 dne: introduce new directory hash type: 'crush'")
Test-Parameters: trivial testlist=sanity env=ONLY=160f,ONLY_REPEAT=50
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I8bead0c20efeda90b9b89f933a022cc18e3ebbe5
Reviewed-on: https://review.whamcloud.com/38129
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>