x86/retbleed: Add call depth tracking mitigation

The fully secure mitigation for RSB underflow on Intel SKL CPUs is IBRS,
which inflicts up to 30% penalty for pathological syscall heavy work loads.

Software based call depth tracking and RSB refill is not perfect, but
reduces the attack surface massively. The penalty for the pathological case
is about 8% which is still annoying but definitely more palatable than IBRS.

Add a retbleed=stuff command line option to enable the call depth tracking
and software refill of the RSB.

This gives admins a choice. IBeeRS are safe and cause headaches, call depth
tracking is considered to be s(t)ufficiently safe.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/20220915111149.029587352@infradead.org