x86/fpu: Cleanup xstate xcomp_bv initialization

x86/fpu: Cleanup xstate xcomp_bv initialization

BugLink: https://bugs.launchpad.net/bugs/1967750
No point in having this duplicated all over the place with needlessly
different defines.

Provide a proper initialization function which initializes user buffers
properly and make KVM use it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.897664678@linutronix.de
(cherry picked from commit 126fe0401883598b45b34dbbd5e0d7d8a0aefa21)
Acked-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>