LU-12142 readahead: limit over reservation

LU-12142 readahead: limit over reservation

For performance reason, exceeding @ra_max_pages are allowed to
cover current read window, but this should be limited with RPC
size in case a large block size read issued. Trim to RPC boundary.

Otherwise, too many read ahead pages might be issued and
make client short of LRU pages.

Fixes: 777b04a093 ("LU-13386 llite: allow current readahead to exceed reservation"
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: Icf74b5fbc75cf836fedcad5184fcdf45c7b037b4
Reviewed-on: https://review.whamcloud.com/42060
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>