UBIFS: dump a little more in case of corruptions

In case of corruptions, dump 8192 bytes instead of 4096. The
largest node is 4096+ bytes, so it is better to see a node
boundary, which is not always possible when only 4096 bytes
are printed.

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