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>