EX-6127 osc: add compression to can_merge_pages

EX-6127 osc: add compression to can_merge_pages

Some BRW flags are OK to have on only some pages in a BRW,
others are not.  can_merge_pages has a whitelist of the
flags which are safe to have on only some pages in a BRW,
and prints a warning if other flags are seen.

Add compression to the white list, because while all pages
in an niobuf must be compressed, it is normal to have only
some pages in a BRW compressed.

Prior to this patch, this warning was printing during
normal usage of compression.

Test-Parameters: trivial
Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: Ia262d4fc878e5328bd956865047e997aa77946f0
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/51528
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>