3.10 API change: block_device_operations->release() returns void

Linux kernel commit torvalds/linux@db2a144 changed the return type
of block_device_operations->release() to void. Detect the expected
prototype and defined our callout accordingly.

Signed-off-by: Chris Dunlop <chris@onthe.net.au>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #1494