EX-7601 ofd: add decompress_read to ofd_preprw_write

EX-7601 ofd: add decompress_read to ofd_preprw_write

We have read up the compressed data from disk, now we must
decompress it so we can rewrite it successfully.

This code still works on the whole lnbs rather than just on
the portion of it which is unaligned.  This is temporary
and will be resolved by a future patch.

With this patch, we have basic read-modify-write support,
so we can re-enable testing.  The next patch adds tests
for read-modify-write.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ib6503c15e9fb3d425a7bc295bcc61b41c089a1f0
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52983
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Reviewed-by: Artem Blagodarenko <ablagodarenko@ddn.com>