selftests/x86: Add test_vsyscall

This tests that the vsyscall entries do what they're expected to do.
It also confirms that attempts to read the vsyscall page behave as
expected.

If changes are made to the vsyscall code or its memory map handling,
running this test in all three of vsyscall=none, vsyscall=emulate,
and vsyscall=native are helpful.

(Because it's easy, this also compares the vsyscall results to their
 vDSO equivalents.)

Note to KAISER backporters: please test this under all three
vsyscall modes.  Also, in the emulate and native modes, make sure
that test_vsyscall_64 agrees with the command line or config
option as to which mode you're in.  It's quite easy to mess up
the kernel such that native mode accidentally emulates
or vice versa.

Greg, etc: please backport this to all your Meltdown-patched
kernels.  It'll help make sure the patches didn't regress
vsyscalls.

CSigned-off-by: Andy Lutomirski <luto@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Hugh Dickins <hughd@google.com>
Cc: Josh Poimboeuf <jpoimboe@redhat.com>
Cc: Juergen Gross <jgross@suse.com>
Cc: Kees Cook <keescook@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: stable@vger.kernel.org
Link: http://lkml.kernel.org/r/2b9c5a174c1d60fd7774461d518aa75598b1d8fd.1515719552.git.luto@kernel.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>