selftests: memfd_test.c: fix compilation warning.

Replace '%d' by '%zu' to fix the following compilation warning.

memfd_test.c:517:3: warning: format ‘%d’ expects argument of
type ‘int’,but argument 2 has type ‘size_t’ [-Wformat=]
   printf("malloc(%d) failed: %m\n", mfd_def_size * 8);
   ^
memfd_test.c: In function ‘mfd_fail_grow_write’:
memfd_test.c:537:3: warning: format ‘%d’ expects argument
of type ‘int’,but argument 2 has type ‘size_t’ [-Wformat=]
   printf("malloc(%d) failed: %m\n", mfd_def_size * 8);

Signed-off-by: Lei Yang <Lei.Yang@windriver.com>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>