x86/fpu/signal: Change return code of check_xstate_in_sigframe() to boolean

__fpu_sig_restore() only needs success/fail information and no detailed
error code.

Suggested-by: Al Viro <viro@zeniv.linux.org.uk>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210908132526.024024598@linutronix.de