x86-64: Slightly shorten copy_page()

%r13 got saved and restored without ever getting touched, so
there's no need to do so.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Link: http://lkml.kernel.org/r/4F05D9F9020000780006AA0D@nat28.tlf.novell.com


Signed-off-by: Ingo Molnar <mingo@elte.hu>