EX-7601 ofd: put decompress_read in read prep

EX-7601 ofd: put decompress_read in read prep

ofd_decompress_read is called from ofd_write_prep for
writes, but from tgt_brw_read for reads.  This makes the
code a little harder to follow and makes it difficult to
check read side decompression against EOF.

Instead, we move the decompression call to ofd_preprw_read.
This makes no change to the real operations here, but makes
for better code (and more similar code between read and
write).

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ibefd0a48ad08e83725f2df64618db60ba61c5ce0
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53427
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>