EX-6265 llite: allow mmap reads of compressed files

EX-6265 llite: allow mmap reads of compressed files

mmap reads of compressed files work, so we should only
block writes.

We cannot block the actual fault operations because that
will cause the application to get a SIGBUS, so we check
the file open mode when we go to create the memory mapping.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I636f398fd247ddcd153f94bf8116440540e8469c
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51506
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>