mtd: nand: don't leak buffers when ->scan_bbt() fails

This bug seems to have been here forever, although we came close to
fixing all of them in [1]!

[1] 11eaf6df ("mtd: nand: Remove BUG() abuse in nand_scan_tail")

Signed-off-by: Brian Norris <computersforpeace@gmail.com>
Acked-by: Ezequiel Garcia <ezequiel@vanguardiasur.com.ar>
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>