x86/fpu: Remove error return values from copy_kernel_to_*regs() functions

None of the copy_kernel_to_*regs() FPU register copying functions are
supposed to fail, and all of them have debugging checks that enforce
this.

Remove their return values and simplify their call sites, which have
redundant error checks and error handling code paths.

Cc: Andy Lutomirski <luto@amacapital.net>
Cc: Bobby Powers <bobbypowers@gmail.com>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Dave Hansen <dave.hansen@linux.intel.com>
Cc: Fenghua Yu <fenghua.yu@intel.com>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>