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

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