block: IBM RamSan 70/80 driver fixes

This patch includes the following driver fixes for the
IBM RamSan 70/80 driver:

o Changed the creg_ctrl lock from a mutex to a spinlock.
o Added a count check for ioctl calls.
o Removed unnecessary casting of void pointers.
o Made every function static that needed to be.
o Added comments to explain things more thoroughly.

Signed-off-by: Philip J Kelleher <pjk1939@linux.vnet.ibm.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>