ceph: queue cap_snaps once per realm

We were forming a dirty list, and then queueing cap_snaps for each realm
_and_ its children, regardless of whether the children were already in the
dirty list.  This meant we did it twice for some realms.  Which in turn
meant we corrupted mdsc->snap_flush_list when the cap_snap was re-added to
the list it was already on, and could trigger an infinite loop.

We were also using recursion to do reach all the children, a no-no when
stack is limited.

Instead, (re)queue any children on the dirty list, avoiding processing
anything twice and avoiding any recursion.

Signed-off-by: Sage Weil <sage@newdream.net>