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

Now that copy_fpregs_to_sigframe() returns boolean the individual return
codes in the related helper functions do not make sense anymore. Change
them to return boolean success/fail.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210908132525.794334915@linutronix.de