Zero out blocks if BLKSECDISCARD fails

On a device where we can't BLKSECDISCARD sectors, we "overwrite" them
with zeroes. This changes the FTL to remap those sectors to new
locations.  With this done, the old contents are accessible only given
a compromise of flash firmware or a die level attack.

Bug: 26021231
Change-Id: Ia065921389886fac1ba456c19c138187237c2561
1 file changed