EX-8421 llite: disable kernel readahead for pcc mmap

EX-8421 llite: disable kernel readahead for pcc mmap

Set ra_pages to 0 for PCC files when mmaped, because
otherwise this setting carries through to Lustre and will
cause crashes and possible inconsistencies.  This happens
because the PCC file and Lustre file share a mapping, which
is a weird trick required to have mmap work on PCC.

Add a set of asserts which confirm kernel readahead is
disabled and wasn't used for mmap.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I117042d68fac25158e8141c243acba698cf1930f
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52732
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Qian Yingjin <qian@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>