ubifs stability work

Use vmalloc when allocating memory to hold an entire eraseblock.
nand_scan_bbt was using vmalloc, but nand_update_bbt was using
kmalloc.

Increase the threshold of corrected bits before returning
UBI_IO_BITFLIPS from 18 to 72.

Change-Id: I6ee9156a3c7efa37e9fd73f1dbe8a6044ae66bad
2 files changed