bpf, lsm: Fix the file_mprotect LSM test.

The test was previously using an mprotect on the heap memory allocated
using malloc and was expecting the allocation to be always using
sbrk(2). This is, however, not always true and in certain conditions
malloc may end up using anonymous mmaps for heap alloctions. This means
that the following condition that is used in the "lsm/file_mprotect"
program is not sufficent to detect all mprotect calls done on heap
memory:

	is_heap = (vma->vm_start >= vma->vm_mm->start_brk &&
		   vma->vm_end <= vma->vm_mm->brk);

The test is updated to use an mprotect on memory allocated on the stack.
While this would result in the splitting of the vma, this happens only
after the security_file_mprotect hook. So, the condition used in the BPF
program holds true.

Fixes: 03e54f10

 ("bpf: lsm: Add selftests for BPF_PROG_TYPE_LSM")
Reported-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: KP Singh <kpsingh@google.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20200402200751.26372-1-kpsingh@chromium.org