LU-949 test: improve sanity test_42e

LU-949 test: improve sanity test_42e

The sanity test_42e should check that the RPCs with less than 16 pages
are not more than 15%, instead of checking the 16-page RPCs are more
than 85%, because the pages could be merged into even larger(> 16 pages)
RPCs.

Signed-off-by: Niu Yawei <niu@whamcloud.com>
Change-Id: I603f9d278b55cd850d66466050745ecb80e2ff02
Reviewed-on: http://review.whamcloud.com/1897
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Yu Jian <yujian@whamcloud.com>