ubifs: Silence early error messages if MS_SILENT is set

Probe-mounting a volume too small for UBIFS results in kernel log
polution which might irritate users.
Address this by silencing errors which may happen during boot if the
rootfs is e.g. squashfs (and thus rather small) stored on a UBI volume.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Signed-off-by: Richard Weinberger <richard@nod.at>