mtd: Account for BBT blocks when a partition is being allocated

With the introduction of mtd_block_isreserved(), it's now possible
to fix the bad and reserved block distribution exposed by ecc_stats,
instead of accounting all the bad or reserved blocks as 'bad'.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Tested-by: Pekon Gupta <pekon@ti.com>
Signed-off-by: Brian Norris <computersforpeace@gmail.com>