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>
Merge pull request #688 from ceph/wip-6474

osd/ReplicatedPG: fix null deref on rollback_to whiteout check

Reviewed-by: Samuel Just <sam.just@inktank.com>