md/bitmap: move setting of daemon_lastrun out of bitmap_read_sb

Setting daemon_lastrun really has nothing to do with reading
the bitmap superblock, it just happens to be needed at the same time.
bitmap_read_sb is about to become options, so move that code out
to after the call to bitmap_read_sb.

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