Revert "x86/ptrace: Prevent ptrace from clearing the FS/GS selector" and fix the...

Revert "x86/ptrace: Prevent ptrace from clearing the FS/GS selector" and fix the test

This reverts commit 599055da67f315948841593098a00c5937402293.

The ptrace ABI change was a prerequisite to the proposed design for
FSGSBASE.  Since FSGSBASE support has been reverted, and since I'm not
convinced that the ABI was ever adequately tested, revert the ABI change as
well.

This also modifies the test case so that it tests the preexisting behavior.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/fca39c478ea7fb15bc76fe8a36bd180810a067f6.1563200250.git.luto@kernel.org