rbd: turn off interrupts for open/remove locking

This commit:
    bc7a62ee5 rbd: prevent open for image being removed
added checking for removing rbd before allowing an open, and used
the same request spinlock for protecting that and updating the open
count as is used for the request queue.

However it used the non-irq protected version of the spinlocks.
Fix that.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>
rbd: add barriers near done flag operations

Somehow, I missed this little item in Documentation/atomic_ops.txt:
    *** WARNING: atomic_read() and atomic_set() DO NOT IMPLY BARRIERS! ***

Create and use some helper functions that include the proper memory
barriers for manipulating the done field.

Signed-off-by: Alex Elder <elder@inktank.com>
Reviewed-by: Josh Durgin <josh.durgin@inktank.com>