EX-7818 osc: don't check for start inside the chunk

EX-7818 osc: don't check for start inside the chunk

Chunk size is the same for the whole request and every
chunk offset is multiple to a chunk size.

No need to search for compression header in every page.
It is enough to check every with offset multiple to a chunk
size.

Test-Parameters: testlist=sanity-compr env=ONLY="1-6"
Signed-off-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ie2ef645130656279e152ea1f7e6db01cb33836ca
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51650
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>