LU-13386 llite: allow current readahead to exceed reservation

LU-13386 llite: allow current readahead to exceed reservation

ll_ra_count_get() is designed to allow over reservation for performance
reasons, this could happen that multiple threads consuming a small
readahead window limit, it will fallback to small 4K read which hurts
performances.

Fixes: 8fbef5e("LU-12043 llite: make sure readahead cover current read")
Change-Id: Ied4ee0ffcd55c568c70f7d3074e810cd0034ef04
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/38047
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>