md: correctly update sysfs when a raid1 is reshaped

When a raid1 array is reshaped (number of drives changed), the list of devices
is compacted, so that slots for missing devices are filled with working
devices from later slots.  This requires the "rd%d" symlinks in sysfs to be
updated.

Signed-off-by: Neil Brown <neilb@suse.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>