backports: rename some mem functions to not break custom kernels

When custom patches are cherry-picked to a kernel, some symbols exported
by backports may clash with the built-in ones. Rename the backports
symbols using the standard backport_ prefix to prevent that.

The offending symbols were exported by the patch below:

commit 2ce5c22448bb45998318267c00b5d6ef9cff3170
Author: Hauke Mehrtens <hauke@hauke-m.de>
Date:   Thu Jun 6 13:48:04 2013 +0200

    backports: backport some memory functions

Signed-off-by: Arik Nemtsov <arik@wizery.com>
Signed-off-by: Luis R. Rodriguez <mcgrof@do-not-panic.com>