MD: ignore discard request for hard disks of hybid raid1/raid10 array

MD: ignore discard request for hard disks of hybid raid1/raid10 array

commit 32f9f570d04461a41bdcd5c1d93b41ebc5ce182a upstream.

In SSD/hard disk hybid storage, discard request should be ignored for hard
disk. We used to be doing this way, but the unplug path forgets it.

This is suitable for stable tree since v3.6.

Reported-and-tested-by: Markus <M4rkusXXL@web.de>
Signed-off-by: Shaohua Li <shli@fusionio.com>
Signed-off-by: NeilBrown <neilb@suse.de>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>