osd/ReplicatedPG: fix null deref on rollback_to whiteout check

Bring this whole if/else chain up one level so that we can capture both 
ENOENT and whiteout in the same case.  (And don't dereference the pointer
when we know it is NULL.)

Fixes: #6474 Signed-off-by: Sage Weil <sage@inktank.com>