kernel: backport cosmetic ubifs patch

kernel: backport cosmetic ubifs patch

Despite the MS_SILENT flag being set when probing for ubifs rootfs a
logline indicating an error is generated during boot:
UBIFS error (pid: 1): cannot open "ubi0:rootfs", error -19
This leads to confusion and there shouldn't be lines containing
the word 'error' twice in a bootlog if actually everything is fine
(just the rootfs happens to be something else than ubifs)

The patch added has been submitted and was accepted upstream, see:
http://lists.infradead.org/pipermail/linux-mtd/2016-June/068056.html
http://patchwork.ozlabs.org/patch/637491

Signed-off-by: Daniel Golle <daniel@makrotopia.org>