EX-7601 obd: do not decompress empty lnbs

EX-7601 obd: do not decompress empty lnbs

For reads which cross EOF, we may get lnbs with no data in
them (similarly for writes which cross EOF).

For these cases, it's important to only copy from the lnbs
where there is data, and only do decompression on the lnbs
if there's actually data in them.

Modify merge chunk to do this.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I83fefcfa6d1396dcd97fad994334bf29438bb4bf
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53430
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>