EX-7601 osc: remove &pga usage in compress_request

EX-7601 osc: remove &pga usage in compress_request

The usage of 'pga' and '&pga' in compress_request is
confusing, but also, compress_request modifies &pga by
allocating a new compressed page array.  Except if we fail
in compress_request, we free that new page array.

This means failing in compress_request replaces 'pga' with
a pointer to freed memory.  Instead, create an explicit
cpga pointer in the caller and use that.  This allows
compress_request to fail safely.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Idaf592103c57b0e9ce76ab520a69b819d4f37be9
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53120
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>