mmc: mmc_test: Fix NULL dereference on allocation failure

If the "test->highmem = alloc_pages()" allocation fails then calling
__free_pages(test->highmem) will result in a NULL dereference.  Also
change the error code to -ENOMEM instead of returning success.

Fixes: 2661081 ("mmc_test: highmem tests")
Signed-off-by: Dan Carpenter <dan.carpenter@linaro.org>
Link: https://lore.kernel.org/r/8c90be28-67b4-4b0d-a105-034dc72a0b31@stanley.mountain
Signed-off-by: Ulf Hansson <ulf.hansson@linaro.org>

Author: Dan Carpenter

drivers/mmc/core/mmc_test.c

@@ -3125,20 +3125,21 @@ static ssize_t mtf_test_write(struct file *file, const char __user *buf,
 	test->buffer = kzalloc(BUFFER_SIZE, GFP_KERNEL);
 #ifdef CONFIG_HIGHMEM
 	test->highmem = alloc_pages(GFP_KERNEL | __GFP_HIGHMEM, BUFFER_ORDER);
+	if (!test->highmem) {
+		count = -ENOMEM;
+		goto free_test_buffer;
+	}
 #endif
 
-#ifdef CONFIG_HIGHMEM
-	if (test->buffer && test->highmem) {
-#else
 	if (test->buffer) {
-#endif
 		mutex_lock(&mmc_test_lock);
 		mmc_test_run(test, testcase);
 		mutex_unlock(&mmc_test_lock);
 	}
 
 #ifdef CONFIG_HIGHMEM
 	__free_pages(test->highmem, BUFFER_ORDER);
+free_test_buffer:
 #endif
 	kfree(test->buffer);
 	kfree(test);