LU-11548 llite: increase readahead default values

LU-11548 llite: increase readahead default values

It is commonly recommended to increase the readahead tunables
for clients to increase performance, since the current defaults
are too small, having been set several years ago for slower
networks and servers.

Increase the readahead defaults to better match values that are
recommended today:
- read_ahead_max_mb increased from 64MB to 1GB by default,
  or 1/32 RAM, whichever is less
- read_ahead_per_file_max_mb is increased from 64MB to 256MB,
  or 1/4 of read_ahead_max_mb, whichever is less

Modify the constant names to better match the variable and /proc
filenames.

Fix sanity test_101g to allow readahead to generate extra read
RPCs, as long as they are the expected size or larger.

Signed-off-by: Andreas Dilger <adilger@whamcloud.com>
Change-Id: Iec864788fa1979c27adad42e613d1bf03f3ebbe5
Reviewed-on: https://review.whamcloud.com/33400
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Wang Shilong <wshilong@whamcloud.com>
Reviewed-by: Yingjin Qian <qian@ddn.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>