floppy: use atomic type for usage_count

The usage_count was being protected by a lock which was only there to
create an atomic counter.

Signed-off-by: Stephen Hemminger <shemminger@vyatta.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Jens Axboe <jaxboe@fusionio.com>