EX-7601 ofd: add chunk_size to preprw_write

EX-7601 ofd: add chunk_size to preprw_write

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

This patch is really trivial on the write side, since the
read side already did most of this.  But it's being kept
separate for symmetry.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Id25957dbc185b6e61b7f208cee8cf5f897f03944
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52962
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>