x86/fpu/xstate: Use fpstate for os_xsave()

x86/fpu/xstate: Use fpstate for os_xsave()

BugLink: https://bugs.launchpad.net/bugs/1967750
With variable feature sets XSAVE[S] requires to know the feature set for
which the buffer is valid. Retrieve it from fpstate.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211013145323.025695590@linutronix.de
(cherry picked from commit 073e627a4537e682c43a1e8df659ce24cbced40c)
Acked-by: Dimitri John Ledkov <dimitri.ledkov@canonical.com>
Signed-off-by: Andrea Righi <andrea.righi@canonical.com>