UBIFS: use EROFS when emulating failures

When the debugging failure emulation is enabled and UBIFS decides to
emulate an I/O error, it uses EIO error code. In which case UBIFS
switches into R/O mode later on. The for the user-space is that when
a failure is emulated, the file-system sometimes returns EIO and
sometimes EROFS. This makes it more difficult to implement user-space
tests for the failure mode. Let's be consistent and return EROFS in
all the cases.

This patch is an improvement for the debugging code and does not affect
the functionality at all.

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