handle_failed_stripe() makes the stripe fail, eg, all IO will return
with a failure, but it doesn't update stripe_head_state. Later
handle_stripe() has special handling for raid6 for handle_stripe_fill().
That check before handle_stripe_fill() doesn't skip the failed stripe
and we get a kernel crash in need_this_block.  This patch clear the
analysis state to make sure no functions wrongly called after
handle_failed_stripe()

Signed-off-by: Shaohua Li <shli@fb.com>
Signed-off-by: NeilBrown <neilb@suse.com>