dm stats: check for and propagate alloc_percpu failure

In the Linux kernel, the following vulnerability has been resolved:

dm stats: check for and propagate alloc_percpu failure

Check alloc_precpu()'s return value and return an error from

dm_stats_init() if it fails. Update alloc_dev() to fail if

dm_stats_init() does.

Otherwise, a NULL pointer dereference will occur in dm_stats_cleanup()

even if dm-stats isn't being actively used.