EX-9878 csdc: is_chunk_start should return header copy

EX-9878 csdc: is_chunk_start should return header copy

In is_chunk_start()

*ret_header = header;
...
kunmap_atomic(header);

ret_header is used after is_chunk_start(). The header
copy should be returned from is_chunk_start() for safe work.

Test-Parameters: testlist=sanity-compr
Signed-off-by: Artem Blagodarenko <ablagodarenko@ddn.com>
Change-Id: Ib5e828d6b61e90dcd70c28589931a4490cf19c22
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/55292
Reviewed-by: Alex Zhuravlev <bzzz@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>