UBIFS: remove unneeded unlikely()

IS_ERR() macro already has unlikely(), so do not use constructions
like 'if (unlikely(IS_ERR())'.

Signed-off-by: Hirofumi Nakagawa <hnakagawa@miraclelinux.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>