LU-14058 tests: handle more MDTs in sanity.sh

LU-14058 tests: handle more MDTs in sanity.sh

Fix up sanity.sh test_160 to handle configurations with more MDTs.
The "fnv_1a_64" hash is _relatively_ uniform and harder to break
under normal (ab)use, it doesn't leave totally entries balanced.
Even "all_chars" hash has a repeat MDT every handful of entries.
Since we need perfect balance across MDTs, use "lfs mkdir -i".

Fix a bug in test_160g that wasn't setting changelog_max_idle_indexes
properly for test systems with more than 4 MDTs.

Lustre-change: https://review.whamcloud.com/41485
Lustre-commit: 173bccd140adf69ce08c20810a69e783c8c12595

Test-Parameters: trivial testlist=sanity env=ONLY=160,230 mdtcount=8
Fixes: 489afbe69d5b ("LU-13321 tests: force even DNE file distribution")
Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: I08bf2274a00fe1c6e52ec1a55f50dc8662d354a9
Reviewed-by: James Nunez <jnunez@whamcloud.com>
Reviewed-by: Lai Siyao <lai.siyao@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/46772
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: John L. Hammond <jhammond@whamcloud.com>