perf/core: Allow kernel address filter when not filtering the kernel

The so-called 'kernel' address filter can also be useful for filtering
fixed addresses in user space.  Allow that.

Signed-off-by: Adrian Hunter <adrian.hunter@intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220131072453.2839535-6-adrian.hunter@intel.com