mtd: nand: erase block before marking bad

Many NAND flash systems (especially those with MLC NAND) cannot be
reliably written twice in a row. For instance, when marking a bad block,
the block may already have data written to it, and so we should attempt
to erase the block before writing a bad block marker to its OOB region.

We can ignore erase failures, since the block may be bad such that it
cannot be erased properly; we still attempt to write zeros to its spare
area.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Signed-off-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Signed-off-by: David Woodhouse <David.Woodhouse@intel.com>