LU-13293 llite: don't abort readahead too aggressively

LU-13293 llite: don't abort readahead too aggressively

We will stop readahead if we have covered @ria_end_idx_min
but lock contention case happen, this could cause a problem
with small read with SSF mode.

Because even lock contention happen, but every client could
potentially have a large consecutive pages to read, if this
is a 4K read, it will end up with a lot small read.

To fix this problem, at least allow readahead continue with
one stripe size, this is exact behavior before the commit

Without Patch:
Max Write: 13082.37 MiB/sec (13717.85 MB/sec)
Max Read:  854.17 MiB/sec (895.67 MB/sec)

With Patch:
Max Write: 12448.90 MiB/sec (13053.61 MB/sec)
Max Read: 23921.73 MiB/sec (25083.75 MB/sec)

Fixes: cfbeae9 ("LU-12043 llite: extend readahead locks for striped file")
Change-Id: I59963592f6dbe6babd746cd01441f4a99a8cafcb
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-on: https://review.whamcloud.com/37697
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>