EX-7601 ofd: switch preprw to chunk_bits

EX-7601 ofd: switch preprw to chunk_bits

The compression/decompression code requires chunk_bits
rather than chunk size.  Since we need to call this code
from ofd_preprw_write, we need chunk_bits there.

This modifies the functions so chunk_bits is available
there.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Id98e6d6364eeaaa7753a8aba059387e3e659d2a2
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52982
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>