dm cache policy smq: change the mutex to a spinlock

We no longer sleep in any of the smq functions, so this can become a
spinlock.  Switching from mutex to spinlock improves performance when
the fast cache device is a very low latency device (e.g. NVMe SSD).

The switch to spinlock also allows for removal of the extra tick_lock;
which is no longer needed since the main lock being a spinlock now
fulfills the locking requirements needed by interrupt context.

Signed-off-by: Joe Thornber <ejt@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

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