EX-7600 osc: use pages_left in unmerge_chunk

EX-7600 osc: use pages_left in unmerge_chunk

Since we have compressed chunks < chunk_size (if they're
after EOF), we must use pages_left in unmgerge_chunk or it
will go off the end of the page array.

This also lets us remove the workaround where unmerge_chunk
would skip pages that were not present.  unmerge_chunk
always works with a known and complete set of pages, so this
check is unneeded.

We should also check that our count of bytes is correct
when we finish.

Signed-off-by: Patrick Farrell <pfarrell@whamcloud.com>
Change-Id: I88896307990ff839514e54e9a7e18390a457e5d8
Reviewed-on: https://review.whamcloud.com/c/ex/lustre-release/+/53095
Tested-by: jenkins <devops@whamcloud.com>
Tested-by: Maloo <maloo@whamcloud.com>
Reviewed-by: Andreas Dilger <adilger@whamcloud.com>