x86/fpu: Rename save_user_xstate() to copy_fpregs_to_sigframe()

Move the naming in line with existing names, so that we now have:

  copy_fpregs_to_fpstate()
  copy_fpstate_to_sigframe()
  copy_fpregs_to_sigframe()

... where each function does what its name suggests.

Cc: Andy Lutomirski <luto@amacapital.net>
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: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Ingo Molnar <mingo@kernel.org>