x86/fpu: Rename xstate copy functions which are related to UABI

Rename them to reflect that these functions deal with user space format
XSAVE buffers.

      copy_kernel_to_xstate() -> copy_uabi_from_kernel_to_xstate()
      copy_user_to_xstate()   -> copy_sigframe_from_user_to_xstate()

Again a clear statement that these functions deal with user space ABI.

Suggested-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210623121454.318485015@linutronix.de