rb://1293
approved by: Marko Makela

There is race when dropping a single table tablespace where a reader
thread can initiate a read request before the delete flag is set and
before it is finished the deleting thread can attempt to free the
fil_node.

This patch checks the status in fil_io() to make sure that the
tablespace is not being deleted. If it is being deleted then
an error is returned instead of attempting IO.