power/perf/hv-24x7: Use kmem_cache_free() instead of kfree

Use kmem_cache_free() to free a buffer allocated with kmem_cache_alloc().

Signed-off-by: Sukadev Bhattiprolu <sukadev@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>