x86/fpu/signal: Change return type of fpu__restore_sig() to boolean

None of the call sites cares about the error code. All they need to know is
whether the function succeeded or not.

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/20210908132525.909065931@linutronix.de