x86/cpu: Probe the behavior of nulling out a segment at boot time

x86/cpu: Probe the behavior of nulling out a segment at boot time

AMD and Intel do different things when writing zero to a segment
selector.  Since neither vendor documents the behavior well and it's
easy to test the behavior, try nulling fs to see what happens.

Signed-off-by: Andy Lutomirski <[email protected]>
Reviewed-by: Borislav Petkov <[email protected]>
Cc: Andy Lutomirski <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: Brian Gerst <[email protected]>
Cc: Denys Vlasenko <[email protected]>
Cc: H. Peter Anvin <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Rudolf Marek <[email protected]>
Cc: Thomas Gleixner <[email protected]>
Link: http://lkml.kernel.org/r/61588ba0e0df35beafd363dc8b68a4c5878ef095.1460075211.git.luto@kernel.org
Signed-off-by: Ingo Molnar <[email protected]>