x86-64: Fix memcpy() to support sizes of 4Gb and above

x86-64: Fix memcpy() to support sizes of 4Gb and above

While currently there doesn't appear to be any reachable in-tree
case where such large memory blocks may be passed to memcpy(),
we already had hit the problem in our Xen kernels. Just like
done recently for mmeset(), rather than working around it,
prevent others from falling into the same trap by fixing this
long standing limitation.

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Link: http://lkml.kernel.org/r/4F21846F020000780006F3FA@nat28.tlf.novell.com
Signed-off-by: Ingo Molnar <mingo@elte.hu>