um: use %lx format specifiers for unsigned longs

static analysis from cppcheck detected %x being used for
unsigned longs:

[arch/x86/um/os-Linux/task_size.c:112]: (warning) %x in format
  string (no. 1) requires 'unsigned int' but the argument type
  is 'unsigned long'.

Use %lx instead of %x

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Richard Weinberger <richard@nod.at>