x86/tls: Validate TLS entries to protect espfix

x86/tls: Validate TLS entries to protect espfix

Installing a 16-bit RW data segment into the GDT defeats espfix.
AFAICT this will not affect glibc, Wine, or dosemu at all.

Signed-off-by: Andy Lutomirski <[email protected]>
Acked-by: H. Peter Anvin <[email protected]>
Cc: [email protected]
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: [email protected] <[email protected]>
Cc: Willy Tarreau <[email protected]>
Signed-off-by: Ingo Molnar <[email protected]>