selftests/openat2: Fix wrong format specifier

Compiling openat2 selftest after adding a __printf() attribute to
ksft_print_msg() exposes a -Wformat warning in test_openat2_flags().
The wrong format specifier is used for printing test.how->flags
variable.

Change the format specifier to %llX so it matches the printed variable.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Reviewed-by: Ilpo Järvinen <ilpo.jarvinen@linux.intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>