LU-14616 readahead: fix reserving for unaliged read

If read is [2K, 3K] on x86 platform, we only need
read one page, but it was calculated as 2 pages.

This could be problem, as we need reserve more
pages credits, vvp_page_completion_read() will only
free actual reading pages, which cause @ra_cur_pages
leaked.

Fixes: d4a54de84c0 ("LU-12367 llite: Fix page count for unaligned reads")
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Change-Id: I3cf03965196c1af833184d9cfc16779f79f5722c
Reviewed-on: https://review.whamcloud.com/43377
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Oleg Drokin <green@whamcloud.com>