x86/vsyscall: Document odd SIGSEGV error code for vsyscalls

Even if vsyscall=none, user page faults on the vsyscall page are reported
as though the PROT bit in the error code was set.  Add a comment explaining
why this is probably okay and display the value in the test case.

While at it, explain why the behavior is correct with respect to PKRU.

Modify also the selftest to print the odd error code so that there is a
way to demonstrate the odd behaviour.

If anyone really cares about more accurate emulation, the behaviour could
be changed. But that needs a real good justification.

Signed-off-by: Andy Lutomirski <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Kees Cook <[email protected]>
Cc: Florian Weimer <[email protected]>
Cc: Jann Horn <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Kernel Hardening <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Link: https://lkml.kernel.org/r/75c91855fd850649ace162eec5495a1354221aaa.1561610354.git.luto@kernel.org

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