ltp: fix mmap15 failed on qemumips64

ltp: fix mmap15 failed on qemumips64

Actually, this is not a bug, mmap15 only run on 64bit system.
On qemumips64, mmap15 return EINVAL, x86-64 and arm64 return
ENOMEM. This is because mips system check the addr that passed
to the syscall mmap15:

    https://github.com/torvalds/linux/blob/master/arch/mips/mm/mmap.c#L71

If the addr larger than (TASK_SIZE - page_size), mips think it is invalid.

Signed-off-by: Dengke Du <dengke.du@windriver.com>
Signed-off-by: Ross Burton <ross.burton@intel.com>