md: write bitmap information to devices that are undergoing recovery.

When we add some spares to an array and start recovery, and we have
a bitmap which is stored 'internally' on all devices, we call
bitmap_write_all to make sure the bitmap is correct on the new
device(s).
However that doesn't work as write_sb_page only writes to
'In_sync' devices, and devices undergoing recovery are not
'In_sync' until recovery finishes.

So extend write_sb_page (actually next_active_rdev) to include devices
that are under recovery.

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