EX-7601 tgt: reorder tgt_brw_write decls

EX-7601 tgt: reorder tgt_brw_write decls

Reorder the declarations in tgt_brw_write.

This patch also serves as the series head for implementing
read-modify-write support for compressed chunks.

The process for read-modify-write is similar to that used
for unaligned reads.

At a high level, read-modify-write means we must read up,
decompress, then recompress and write back the data.  This
only applies when we're actually doing read-modify-write.

To know when to do this, we rely partly on the client.  If
the client is able to compress a chunk, either because it is
a complete chunk, or because the start is chunk aligned and
the write is past EOF, we know there is no read-modify-write
required.  Either there is no existing data (write past EOF)
or the data will be fully replaced.

So, when we see a write which is not fully chunk aligned and
not already compressed, we will do a read-modify-write.

For this, we round the IO lnbs and associated locking to
cover complete chunks, then we do a read of the unaligned
chunks.

ie, if we have a write which goes from 63 KiB to 257 KiB
with a chunk size of 64 KiB, we will read 0-64 KiB and
256-320 KiB, and decompress those chunks in to the buffer.
64 KiB to 256 KiB is *NOT* read, because those are complete
chunks.

We then set up a transfer mapping - identical to the process
for unaligned reads - so the client data is written in to
the correct lnbs.

Now we have a set of chunk aligned lnbs which contain data
updated with the client write.  In the initial version, we
write these to disk uncompressed.  This is sufficient for
correct operation, but it does mean read-modify-write will
decompress those chunks.

There is code for recompression, but it is not working 100%
yet, and there are some complexities around managing holes
and EOF which still need to be resolved.

TBD if this will make our initial release - I am hopeful but
not sure yet.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ia24583d4221f498928e99afa8c289b70e4d25f5b
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52959
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>