samples: Fix warning in fsnotify sample

The fsnotify sample code generates the following warning on powerpc:

samples/fanotify/fs-monitor.c: In function 'handle_notifications':
samples/fanotify/fs-monitor.c:68:36: warning: format '%llx' expects argument of type 'long long unsigned int', but argument 2 has type '__u64' {aka 'long unsigned int'} [-Wformat=]
   68 |    printf("unexpected FAN MARK: %llx\n", event->mask);
      |                                 ~~~^     ~~~~~~~~~~~
      |                                    |          |
      |                                    |          __u64 {aka long unsigned int}
      |                                    long long unsigned int
      |                                 %lx

Fix the problem by explicitely typing the argument to proper type.

Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Jan Kara <jack@suse.cz>