UBIFS: check ubifs_scan error codes better

The 'ubifs_scan()' function returns -EUCLEAN if something is corrupted
and recovery is needed, otherwise it returns other error codes. However,
in few places UBIFS does not check the error codes and runs recovery.
This patch changes this behavior and makes UBIFS start recovery only
on -EUCLEAN errors.

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