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

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.

---
Changelog:
v2:
- typecast '&reg.write_index' to 'void *' & remove use of memcpy as suggested by Andrew.

v1: https://lore.kernel.org/linux-kselftest/[email protected]/

---
 tools/testing/selftests/user_events/perf_test.c | 2 +-
 1 file changed, 1 insertion(+), 1 deletion(-)

index 201459d8094d..cafec0e52eb3 100644
@@ -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);

--
2.51.0