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: 2661081f ("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.mountainSigned-off-by: Ulf Hansson <ulf.hansson@linaro.org>