selftests/mm: Substitute attribute with a macro

Compiling mm selftest after adding a __printf() attribute to
ksft_print_msg() exposes -Wformat warning in remap_region().

Fix the wrong format specifier causing the warning.

The mm selftest uses the printf attribute in its full form. Since the
header file that uses it also includes kselftests.h it can use the macro
defined there.

Use __printf() included with kselftests.h instead of the full attribute.

Signed-off-by: Maciej Wieczor-Retman <maciej.wieczor-retman@intel.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>