perf: Mark expected switch fall-through

In preparation to enabling -Wimplicit-fallthrough, mark switch cases
where we are expecting to fall through.

This patch fixes the following warning:

  kernel/events/core.c: In function ‘perf_event_parse_addr_filter’:
  kernel/events/core.c:9154:11: warning: this statement may fall through [-Wimplicit-fallthrough=]
      kernel = 1;
      ~~~~~~~^~~
  kernel/events/core.c:9156:3: note: here
     case IF_SRC_FILEADDR:
     ^~~~

Warning level 3 was used: -Wimplicit-fallthrough=3

This patch is part of the ongoing efforts to enable -Wimplicit-fallthrough.
Signed-off-by: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Alexander Shishkin <alexander.shishkin@linux.intel.com>
Cc: Gustavo A. R. Silva <gustavo@embeddedor.com>
Cc: Jiri Olsa <jolsa@redhat.com>
Cc: Kees Kook <keescook@chromium.org>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Link: http://lkml.kernel.org/r/20190212205430.GA8446@embeddedorSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

kernel/events/core.c

@@ -9175,6 +9175,7 @@ perf_event_parse_addr_filter(struct perf_event *event, char *fstr,
 		case IF_SRC_KERNELADDR:
 		case IF_SRC_KERNEL:
 			kernel = 1;
+			/* fall through */
 
 		case IF_SRC_FILEADDR:
 		case IF_SRC_FILE: