Take advantage of the checking provided by refcount_t, rather than
using a plain atomic to represent the IPA clock reference count.

Note that we need to *set* the value to 1 in ipa_clock_get() rather
than incrementing it from 0 (because doing that is considered an
error for a refcount_t).
Signed-off-by: Alex Elder <elder@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>