UBIFS: set write-buffer timout to 3-5 seconds

This patch cleans up write-buffer timeout initialization and
sets it to 3-5 interval.

Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>

diff --cc fs/ubifs/io.c
Simple merge

diff --cc fs/ubifs/ubifs.h
Simple merge