mtd: inftlcore: Add error check for inftl_read_oob()

In INFTL_findwriteunit(), the return value of inftl_read_oob()
need to be checked. A proper implementation can be
found in INFTL_deleteblock(). The status will be set as
SECTOR_IGNORE to break from the while-loop correctly
if the inftl_read_oob() fails.

Fixes: 8593fbc6 ("[MTD] Rework the out of band handling completely")
Cc: stable@vger.kernel.org # v2.6+
Signed-off-by: Wentao Liang <vulab@iscas.ac.cn>
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>