x86/speculation: Protect against userspace-userspace spectreRSB

The article "Spectre Returns! Speculation Attacks using the Return Stack 
Buffer" [1] describes two new (sub-)variants of spectrev2-like attacks, 
making use solely of the RSB contents even on CPUs that don't fallback to 
BTB on RSB underflow (Skylake+).

Mitigate userspace-userspace attacks by always unconditionally filling RSB on
context switch when the generic spectrev2 mitigation has been enabled.

[1] https://arxiv.org/pdf/1807.07940.pdf

Signed-off-by: Jiri Kosina <[email protected]>
Signed-off-by: Thomas Gleixner <[email protected]>
Reviewed-by: Josh Poimboeuf <[email protected]>
Acked-by: Tim Chen <[email protected]>
Cc: Konrad Rzeszutek Wilk <[email protected]>
Cc: Borislav Petkov <[email protected]>
Cc: David Woodhouse <[email protected]>
Cc: Peter Zijlstra <[email protected]>
Cc: Linus Torvalds <[email protected]>
Cc: [email protected]
Link: https://lkml.kernel.org/r/[email protected]

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