x86/pkeys: Add self-tests

This code should be a good demonstration of how to use the new
system calls as well as how to use protection keys in general.

This code shows how to:
1. Manipulate the Protection Keys Rights User (PKRU) register
2. Set a protection key on memory
3. Fetch and/or modify PKRU from the signal XSAVE state
4. Read the kernel-provided protection key in the siginfo
5. Set up an execute-only mapping

There are currently 13 tests:

  test_read_of_write_disabled_region
  test_read_of_access_disabled_region
  test_write_of_write_disabled_region
  test_write_of_access_disabled_region
  test_kernel_write_of_access_disabled_region
  test_kernel_write_of_write_disabled_region
  test_kernel_gup_of_access_disabled_region
  test_kernel_gup_write_to_write_disabled_region
  test_executing_on_unreadable_memory
  test_ptrace_of_child
  test_pkey_syscalls_on_non_allocated_pkey
  test_pkey_syscalls_bad_args
  test_pkey_alloc_exhaust

Each of the tests is run with plain memory (via mmap(MAP_ANON)),
transparent huge pages, and hugetlb.

Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: linux-arch@vger.kernel.org
Cc: Dave Hansen <dave@sr71.net>
Cc: mgorman@techsingularity.net
Cc: arnd@arndb.de
Cc: linux-api@vger.kernel.org
Cc: shuahkh@osg.samsung.com
Cc: linux-mm@kvack.org
Cc: luto@kernel.org
Cc: akpm@linux-foundation.org
Cc: torvalds@linux-foundation.org
Link: http://lkml.kernel.org/r/20160729163024.FC5A0C2D@viggo.jf.intel.com


Signed-off-by: Thomas Gleixner <tglx@linutronix.de>