ubifs: Silence error output if MS_SILENT is set

This change completes commit
90bea5a3 ("UBIFS: respect MS_SILENT mount flag")
which already implements support for MS_SILENT except for that one
error message which is still being displayed despite MS_SILENT being
set. Suppress that error message as well in case MS_SILENT is set.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
[rw: massaged commit message]
Signed-off-by: Richard Weinberger <richard@nod.at>