LU-12043 llite,readahead: don't always use max RPC size

LU-12043 llite,readahead: don't always use max RPC size

Since 64M RPC landed, @PTLRPC_MAX_BRW_PAGES will be 64M.
And we always try to use this max possible RPC size to check
whether we should avoid fast IO and trigger real context IO.

This is not good for following reasons:

(1) Since current default RPC size is still 4M,
most of system won't use 64M for most of time.

(2) Currently default readahead size per file is still 64M,
which makes fast IO always run out of all readahead pages
before next IO. This breaks what users really want for readahead
grapping pages in advance.

To fix this problem, we use 16M as a balance value if RPC smaller
than 16M, patch also fix the problem that @ras_rpc_size could not
grow bigger which is possibe in the following case:

1) set RPC to 16M
2) Set RPC to 64M

In the current logic ras->ras_rpc_size will be kept as 16M which is wrong.

Lustre-change: https://review.whamcloud.com/35033
Lustre-commit: 7864a6854c3dfe3319dcf6809e728eed9a37b9b4

Change-Id: Ida9f839f7c692cd88d32dc0909503f6ae991d909
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Li Xi <lixi@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Signed-off-by: Minh Diep <mdiep@whamcloud.com>
Reviewed-on: https://review.whamcloud.com/35559
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>