x86/events/amd/iommu: Fix sizeof mismatch

An incorrect sizeof is being used, struct attribute ** is not correct,
it should be struct attribute *. Note that since ** is the same size as
* this is not causing any issues.  Improve this fix by using sizeof(*attrs)
as this allows us to not even reference the type of the pointer.

Addresses-Coverity: ("Sizeof not portable (SIZEOF_MISMATCH)")
Fixes: 51686546 ("x86/events/amd/iommu: Fix sysfs perf attribute groups")
Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20201001113900.58889-1-colin.king@canonical.com