librbd: add more robust retry handling to maintenance ops

librbd: add more robust retry handling to maintenance ops

When image locking is enabled, snapshot create, resize, and
flatten are coordinated with the lock owner.  Previously, if the
the lock owner changed during one of this operations, the
operation would fail.  Now librbd will attempt to restart the
operation with the new lock owner (or become the owner itself).

Signed-off-by: Jason Dillaman <dillaman@redhat.com>