EX-7601 tgt: add io_lnb_to_tx_lnb

EX-7601 tgt: add io_lnb_to_tx_lnb

With compression, the lnbs used for the disk IO on the
server can contain more data than the client requested,
due to reading up whole chunks for decompression.

This means we need to transfer only a subset of the lnbs.
We do this by creating a second set of lnbs, and pointing
them at the pages in the local io lnb which need to be
transferred to the client.

This code doesn't do anything for now, but it will kick in
with the next patch when we start rounding chunks for read.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I0fe690718a3484578b139eaaec52c0c3b265da6a
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/52884
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>