Fixed reference to overlapping (the same) string.

The intent of the rootdev_get_device_slave is to allow the slave and
the device to be the same string but it is not required. Added a
test for the case where they are the same string to not do the
strncpy.

Linux has vixed the problem of a limited number of stacked devices,
so I fixed the comment that referred to it.

BUG=chromium:259893
TEST=rootdev -s

Change-Id: I04d84799e77c678f1330e1bcf72cddaf9ac68c4c
Reviewed-on: https://gerrit.chromium.org/gerrit/61707


Tested-by: Paul Taysom <taysom@chromium.org>
Reviewed-by: Yunlian Jiang <yunlian@chromium.org>
Commit-Queue: Paul Taysom <taysom@chromium.org>