UBIFS: fix available blocks count

Take into account that 2 eraseblocks are never available because
they are reserved for the index. This gives more realistic count
of FS blocks.

To avoid future confusions like this, introduce a constant.

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

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

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

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