mtd: nand: rename create_bbt()'s 'len' variable to 'numpages'

Rename 'len' variable of create_bbt/scan_block_fast/scan_block_full to
'numpages', since it really means number of pages to scan when
searching for the BBM (and not the byte length of the scan).

Signed-off-by: Shmulik Ladkani <shmulik.ladkani@gmail.com>
Reviewed-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>