syscalls/fork: add new case fork14

This testcase is a reproducer for https://lkml.org/lkml/2012/4/24/328.[1]
Modified from Siddhesh Poyarekar's testcase posted on above link.
Since vma length in dup_mmap is calculated and stored in a unsigned
int, which is insufficient and hence overflows for very large maps
(beyond 16TB). Once overflow occurred, the fork after mmaped memory >
16TB will succeed incorrectly.

    This case will run following loop:
             + mmap one (more) GB memory
             + fork and check return value.
    When mmaped more than 16 * 1024 GB, it will check if fork still fail.
    Expected result: Fork failed even if mmaped memory > 16 * 1024 GB

The issue has been fixed by:
  commit 7edc8b0ac16cbaed7cb4ea4c6b95ce98d2997e84
  Author: Siddhesh Poyarekar <siddhesh.poyarekar@gmail.com>
    mm/fork: fix overflow in vma length when copying mmap on clone

Signed-off-by: Madper Xie <cxie@redhat.com>