LU-12495 test: shorten qos_maxage to update statfs

LU-12495 test: shorten qos_maxage to update statfs

sanity test_413b() should shorten lmv->desc.qos_maxage to update
cached statfs in time.

Test-Parameter: trivial envdefinitions=ONLY=413b
Test-Parameter: testlist=sanity,sanity,sanity,sanity,sanity,sanity

Signed-off-by: Lai Siyao <lai.siyao@whamcloud.com>
Change-Id: I58672590669be5eaa5c0d679c51cb6cd533bc0d7
Reviewed-on: https://review.whamcloud.com/35395
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Hongchao Zhang <hongchao@whamcloud.com>