mmc_block: Allow more than 8 partitions per card

Set the GENHD_FL_EXT_DEVT flag, which will allocate minor numbers
in major 259 for partitions past disk->minors.

Also remove the use of disk_devt to determine devidx from md->disk.
md->disk->first_minor is always initialized from devidx and can
always be used to recover it.

Signed-off-by: Colin Cross <ccross@android.com>
diff --git a/drivers/mmc/card/block.c b/drivers/mmc/card/block.c
index f0bc6aa..ac13768 100644
--- a/drivers/mmc/card/block.c
+++ b/drivers/mmc/card/block.c
@@ -126,11 +126,7 @@
 
 static inline int mmc_get_devidx(struct gendisk *disk)
 {
-	int devmaj = MAJOR(disk_devt(disk));
-	int devidx = MINOR(disk_devt(disk)) / perdev_minors;
-
-	if (!devmaj)
-		devidx = disk->first_minor / perdev_minors;
+	int devidx = disk->first_minor / perdev_minors;
 	return devidx;
 }
 
@@ -1050,6 +1046,7 @@
 	md->disk->queue = md->queue.queue;
 	md->disk->driverfs_dev = parent;
 	set_disk_ro(md->disk, md->read_only || default_ro);
+	md->disk->flags = GENHD_FL_EXT_DEVT;
 
 	/*
 	 * As discussed on lkml, GENHD_FL_REMOVABLE should: