[MTD] nandsim: enhance nandsim to simulate flash errors

New module parameters have been added to nandsim to
simulate:

        bitflips        random bit flips
        badblocks       blocks that are initially marked bad
        weakblocks      blocks that fail to erase after a
                        small number of erase cycles
        weakpages       pages that fail to write after a
                        small number of successful writes
        gravepages      pages that fail to read after a
                        small number of successful reads
Signed-off-by: Adrian Hunter <ext-adrian.hunter@nokia.com>
Signed-off-by: Artem Bityutskiy <Artem.Bityutskiy@nokia.com>
Signed-off-by: David Woodhouse <dwmw2@infradead.org>