LU-983 llite: align readahead to 1M after ra_max adjustment

LU-983 llite: align readahead to 1M after ra_max adjustment

1. Align the readahead pages only if ria_start != 0, otherwise
   the readahead pages will be cut to zero.
2. Add test_101e to verify small reads for small size files.
   Add test_101f to verify max_read_ahead_whole_mb.
3. Port 101c from b1_8

Signed-off-by: Wang Di <di.wang@whamcloud.com>
Change-Id: I8b31621f5b48c3bc98f8d888f91d65b945c51999
Reviewed-on: http://review.whamcloud.com/2260
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Hudson
Tested-by: Maloo <whamcloud.maloo@gmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>