osd/ReplicatedPG: fix whiteouts for other cache mode

We were special casing WRITEBACK mode for handling whiteouts; this needs to
also include the FORWARD and READONLY modes.  To avoid having to list
specific cache modes, though, just check != NONE.

Fixes: #8296
Backport: firefly
Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #1774 from ceph/wip-8296

osd/ReplicatedPG: fix whiteouts for other cache mode

Reviewed-by: Samuel Just <sam.just@inktank.com>