EX-7601 ofd: add read to write process

EX-7601 ofd: add read to write process

This adds a very simple read to the write process, which
just reads up the entire chunk-rounded write range.

This is a first step - the read will eventually be modified
to only read the unaligned portions which must be
decompressed for read-modify-write.  We will create a
special lnb mapping which contains only the pages which must
be read for decompression (similar to the tx lnb mapping).

For now, this read allows us to test decompression without
handling the mapping.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I169ddc2e161094aebdad1a60ec62e9c1d75cd6d8
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52966
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>