md: Support write-intent bitmaps with externally managed metadata.

In this case, the metadata needs to not be in the same
sector as the bitmap.
md will not read/write any bitmap metadata.  Config must be
done via sysfs and when a recovery makes the array non-degraded
again, writing 'true' to 'bitmap/can_clear' will allow bits in
the bitmap to be cleared again.

Signed-off-by: NeilBrown <neilb@suse.de>

drivers/md/md.h

@@ -296,6 +296,7 @@ struct mddev_s
 		unsigned long		chunksize;
 		unsigned long		daemon_sleep; /* how many seconds between updates? */
 		unsigned long		max_write_behind; /* write-behind mode */
+		int			external;
 	} bitmap_info;
 
 	struct list_head		all_mddevs;