LU-8465 e2fsck: make default smallest RA size to 1M

LU-8465 e2fsck: make default smallest RA size to 1M

If we have a smaller inodes per group, default ra size could
be very small(etc 128KiB), this hurts performances.

Tune above 128K to 1M, i see pass1 time drop down from
677.12 seconds to 246 secons with 32 threads.

Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: Id54e156d3a5c65beee23533aa2ae71d34d65ca20
Reviewed-on: https://review.whamcloud.com/39914
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>