selftests/mount_setattr: fix to make run_tests failure

make run_tests doesn't run the test. Fix Makefile to set TEST_GEN_PROGS
instead of TEST_GEN_FILES to fix the problem.

run_tests runs TEST_GEN_PROGS, TEST_CUSTOM_PROGS, and TEST_PROGS.
TEST_GEN_FILES is for files generated by tests.

Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>