e2fsck: make default smallest RA size to 1M

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.

E2fsprogs-commit: 9226b3264b74044013f4933280b1989c33be42f3

Change-Id: Ia503b4f73dc8f3d5e6be705c7311bdacc5b874a9
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>