mtd: nand_bbt: fix theoretical integer overflow in BBT write

This statement was written with a cast-to-loff_t to be sure to have a
full 64-bit mask. However, we don't account for the fact that
'1 << this->bbt_erase_shift' might already overflow.

This will not be a problem in practice, since eraseblocks should never
be anywhere near 4GiB. But we can do this for completeness, and quiet
Coverity in the meantime. CID #1226806.

Signed-off-by: Brian Norris <computersforpeace@gmail.com>