mtd: rawnand: Fix a memory leak bug

In nand_scan_bbt(), a temporary buffer 'buf' is allocated through
vmalloc(). However, if check_create() fails, 'buf' is not deallocated,
leading to a memory leak bug. To fix this issue, free 'buf' before
returning the error.

Signed-off-by: Wenwen Wang <wenwen@cs.uga.edu>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>