tracing/filters: Fix double-free of struct filter_pred.mask

tracing/filters: Fix double-free of struct filter_pred.mask

When a cpulist filter is found to contain a single CPU, that CPU is saved
as a scalar and the backing cpumask storage is freed.

Also NULL the mask to avoid a double-free once we get down to
free_predicate().

Link: https://lkml.kernel.org/r/20230901151039.125186-3-vschneid@redhat.com
Cc: Masami Hiramatsu <mhiramat@kernel.org>
Reported-by: Steven Rostedt <rostedt@goodmis.org>
Signed-off-by: Valentin Schneider <vschneid@redhat.com>
Signed-off-by: Steven Rostedt (Google) <rostedt@goodmis.org>