Fix bad backport to stable v3.16+

Hi,

As reported by Thorsten Knabe linux@thorsten-knabe.de.

commit 4f4fd7c5798b ("Don't jump to compute_result state from check_result state") was back ported to v3.16+. However, this fix was wrong.

Please back port the following two commits to fix this issue.

commit a25d8c327bb4 ("Revert "Don't jump to compute_result state from check_result state"") commit b2176a1dfb51 ("md/raid: raid5 preserve the writeback action after the parity check")

Thanks, Song