[RFC] MTD API rework: Harmonize initialization of erase_info->fail_addr

[Shmulik Ladkani]

Prior MTD API rework, 'mtd->erase()' users needed to initialize
'erase_info->fail_addr' to MTD_FAIL_ADDR_UNKNOWN.
Sometimes it was taken care of by the mtd driver's 'erase'
implementation.

Harmonize this, by initializing 'fail_addr' within the new 'mtd_erase'
interface.

If idea is suitable, can work on a patch (although not immediately).

Regards,
Shmulik