tests: fix rmmod of test kmod

Thanks to commit c08f6be ("vmtest: kbuild: add
CONFIG_MODULE_UNLOAD=y"), I was finally able to try unloading the test
kernel module and found a trivial copy-and-paste error. Fix it.

Fixes: 42e7d47 ("drgn.helpers.linux.mm: add compound page helpers")
Signed-off-by: Omar Sandoval <osandov@osandov.com>

tests/linux_kernel/kmod/drgn_test.c

@@ -139,7 +139,7 @@ static int drgn_test_mm_init(void)
 static void drgn_test_mm_exit(void)
 {
 	if (drgn_test_compound_page)
-		__free_pages(drgn_test_page, 1);
+		__free_pages(drgn_test_compound_page, 1);
 	if (drgn_test_page)
 		__free_pages(drgn_test_page, 0);
 }