EX-7601 ofd: add chunk_size to preprw_read

EX-7601 ofd: add chunk_size to preprw_read

preprw_read needs chunk size for rounding.  Add this in a
separate patch to keep things trivial, it will be used in
a subsequent patch.

Also use this to add a check in DOM to ensure it doesn't
attempt to do compression.  This should already be
prevented by setstripe, so this is just an extra safety
check.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I9dc4d1559e5c8be315268a593466571b54c90a96
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52866
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>