mds: fix assert(omap_num_objs <= MAX_OBJECTS) of OpenFileTable

mds: fix assert(omap_num_objs <= MAX_OBJECTS) of OpenFileTable

Current behavior of the loop that commits dirty items is:

- If the last object is not full, it will be chosen for new item.
- If the last object becomes full, a new object is allocated, which
  becomes the new last object.

Above logical can make object count increase even there is free object.

Fixes: https://tracker.ceph.com/issues/36094
Signed-off-by: "Yan, Zheng" <zyan@redhat.com>