mtd: inftlcore: Add error check for inftl_read_oob()

In the Linux kernel, the following vulnerability has been resolved:

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.