OSD: check for splitting when processing recover/backfill reservations

Fixes: 6565
Signed-off-by: Samuel Just <sam.just@inktank.com>
Reviewed-by: Sage Weil <sage@inktank.com>
(cherry picked from commit 15ec5332ba4154930a0447e2bcf1acec02691e97)
Merge pull request #1827 from ceph/wip-6565-dumpling

Wip 6565 dumpling

Reviewed-by: Sage Weil <sage@inktank.com>