selftests/user_events: fix type cast for write_index packed member in perf_test

commit 216158f upstream.

Accessing 'reg.write_index' directly triggers a -Waddress-of-packed-member
warning due to potential unaligned pointer access:

perf_test.c:239:38: warning: taking address of packed member 'write_index'
of class or structure 'user_reg' may result in an unaligned pointer value
[-Waddress-of-packed-member]
  239 |         ASSERT_NE(-1, write(self->data_fd, &reg.write_index,
      |                                             ^~~~~~~~~~~~~~~

Since write(2) works with any alignment. Casting '&reg.write_index'
explicitly to 'void *' to suppress this warning.

Link: https://lkml.kernel.org/r/20251106095532.15185-1-ankitkhushwaha.linux@gmail.com
Fixes: 42187bd ("selftests/user_events: Add perf self-test for empty arguments events")
Signed-off-by: Ankit Khushwaha <ankitkhushwaha.linux@gmail.com>
Cc: Beau Belgrave <beaub@linux.microsoft.com>
Cc: "Masami Hiramatsu (Google)" <mhiramat@kernel.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: sunliming <sunliming@kylinos.cn>
Cc: Wei Yang <richard.weiyang@gmail.com>
Cc: Shuah Khan <shuah@kernel.org>
Cc: <stable@vger.kernel.org>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Author: ankitkhushwaha

tools/testing/selftests/user_events/perf_test.c

@@ -236,7 +236,7 @@ TEST_F(user, perf_empty_events) {
 	ASSERT_EQ(1 << reg.enable_bit, self->check);
 
 	/* Ensure write shows up at correct offset */
-	ASSERT_NE(-1, write(self->data_fd, &reg.write_index,
+	ASSERT_NE(-1, write(self->data_fd, (void *)&reg.write_index,
 					sizeof(reg.write_index)));
 	val = (void *)(((char *)perf_page) + perf_page->data_offset);
 	ASSERT_EQ(PERF_RECORD_SAMPLE, *val);