userfaultfd/selftests: fix uninitialized_var.cocci warning

Fix following coccicheck warning:
tools/testing/selftests/vm/userfaultfd.c:556:23-24:
WARNING this kind of initialization is deprecated

`unsigned long page_nr = *(&page_nr)` has the same form of
uninitialized_var() macro. I remove the redundant assignement. It has
been tested with gcc (Debian 8.3.0-6) 8.3.0.

The patch which removed uninitialized_var() is:
https://lore.kernel.org/all/20121028102007.GA7547@gmail.com/ And there is
very few "/* GCC */" comments in the Linux kernel code now.

Link: https://lkml.kernel.org/r/20220304082333.9252-1-guozhengkui@vivo.comSigned-off-by: Guo Zhengkui <guozhengkui@vivo.com>
Cc: Shuah Khan <shuah@kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

tools/testing/selftests/vm/userfaultfd.c

@@ -540,7 +540,7 @@ static void continue_range(int ufd, __u64 start, __u64 len)
 static void *locking_thread(void *arg)
 {
 	unsigned long cpu = (unsigned long) arg;
-	unsigned long page_nr = *(&(page_nr)); /* uninitialized warning */
+	unsigned long page_nr;
 	unsigned long long count;
 
 	if (!(bounces & BOUNCE_RANDOM)) {