selftests/x86/test_vsyscall: Improve the process_vm_readv() test

The existing code accepted process_vm_readv() success or failure as long
as it didn't return garbage.  This is too weak: if the vsyscall page is
readable, then process_vm_readv() should succeed and, if the page is not
readable, then it should fail.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Cc: x86@kernel.org
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: John Hubbard <jhubbard@nvidia.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>