libceph: set 'exists' flag for newly up osd

[ Upstream commit 6dd74e44dc1df85f125982a8d6591bc4a76c9f5d ]

Signed-off-by: Yan, Zheng <zyan@redhat.com>
Reviewed-by: Sage Weil <sage@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>
Signed-off-by: Sasha Levin <alexander.levin@verizon.com>