Fix deviceseeks.stp example to explicitly cast queue variable

Fix deviceseeks.stp example to explicitly cast queue variable

The deviceseeks.stp example was failing to build because several
uses of the queue variable were not explicitly casted and result
in the following error message:

semantic error: autocast variable '' may not be used as a structure: operator '->' at testsuite/systemtap.examples/io/deviceseeks.stp:26:8
        source: 		queue->limits->logical_block_size :
                		     ^
Used @q_cast(queue) in place of the plain queue to correctly
cast the variable and eliminate the error.