md/raid5: fix IO hang with degraded array with llbitmap

In the Linux kernel, the following vulnerability has been resolved: md/raid5: fix IO hang with degraded array with llbitmap When llbitmap bit state is still unwritten, any new write should force rcw, as bitmap_ops->blocks_synced() is checked in handle_stripe_dirtying(). However, later the same check is missing in need_this_block(), causing stripe to deadloop during handling because handle_stripe() will decide to go to handle_stripe_fill(), meanwhile need_this_block() always return 0 and nothing is handled.