LU-14616 readahead: export pages directly without RA

LU-14616 readahead: export pages directly without RA

With Readahead disabled, @vpg_defer_uptodate should not
be set as we don't reserve credits for such read.
In vvp_page_completion_read() we will call ll_ra_count_put()
which makes @ra_cur_pages negative.

Lustre-change: https://review.whamcloud.com/43338
Lustre-commit: 9f1c0bfd10d619a3755c3b22b1dd95a593720ce9

Fixes: 7e8efb339b ("LU-12043 llite: fix to submit complete read block with ra disabled")
Change-Id: I1c9134f5972aa0d0e7aac998f02c690cc55b433b
Signed-off-by: Wang Shilong <wshilong@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Bobi Jam <bobijam@hotmail.com>
Reviewed-on: https://review.whamcloud.com/44246
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>