block: wait_for_overlapping_requests() deadlock detection

Debugging a reentrant request deadlock was fun but in the future we need
a quick and obvious way of detecting such bugs.  Add an assert that
checks we are not about to deadlock when waiting for another request.

Suggested-by: Kevin Wolf <kwolf@redhat.com>
Signed-off-by: Stefan Hajnoczi <stefanha@linux.vnet.ibm.com>
Signed-off-by: Kevin Wolf <kwolf@redhat.com>
