UBIFS: constify operations

Mark super, file, and inode operation structcutes with 'const'.

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

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

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

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

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