If commit failed and it is in broken state, UBIFS switches to R/O mode. Most
operations return -EROFS in this case, except of commit which returns -EINVAL.
Make it return -EROFS too for consistency. This is also important for our power
cut emulation testing.
Signed-off-by: NArtem Bityutskiy <Artem.Bityutskiy@nokia.com>