mtd: Unconditionally update ->fail_addr and ->addr in part_erase()

->fail_addr and ->addr can be updated no matter the result of
parent->_erase(), we just need to remove the code doing the same thing
in mtd_erase_callback() to avoid adjusting those fields twice.

Note that this can be done because all MTD users have been converted to
not pass an erase_info->callback() and are thus only taking the
->addr_fail and ->addr fields into account after part_erase() has
returned.

While we're at it, get rid of the erase_info->mtd field which was only
needed to let mtd_erase_callback() get the partition device back.

Signed-off-by: Boris Brezillon <boris.brezillon@bootlin.com>
Reviewed-by: Richard Weinberger <richard@nod.at>

drivers/mtd/tests/mtd_test.c

@@ -14,7 +14,6 @@ int mtdtest_erase_eraseblock(struct mtd_info *mtd, unsigned int ebnum)
 	loff_t addr = (loff_t)ebnum * mtd->erasesize;
 
 	memset(&ei, 0, sizeof(struct erase_info));
-	ei.mtd  = mtd;
 	ei.addr = addr;
 	ei.len  = mtd->erasesize;