swap: don't do discard if no discard option added

[William Dauchy]

Hello,

On Xen, when booting a guest with a system disk and an additional swap
disk I'm getting a calltrace.
xen hypervisor: 4.1.2; linux dom0: v3.3.6; linux guest: v3.2.17
When booting without a swap disk, I don't have the issue.
I also tested a guest with v3.3.6: same problem. But from v3.4-rc2,
the issue is fixed.
I cherry-picked:
052b198 swap: don't do discard if no discard option added
Applied and tested on top of v3.2.17 and v3.3.6, it fixes the issue.

Pid: 0, comm: swapper/0 Not tainted 3.2.17-x86_64 #12
Call Trace:
 <IRQ>
 [<ffffffff810919da>] ? handle_irq_event_percpu+0x3a/0x140
 [<ffffffff81091b29>] ? handle_irq_event+0x49/0x80
 [<ffffffff81094e7d>] ? handle_edge_irq+0x6d/0x120
 [<ffffffff81229088>] ? __xen_evtchn_do_upcall+0x1b8/0x280
 [<ffffffff8122a442>] ? xen_evtchn_do_upcall+0x22/0x40
 [<ffffffff8133f4fe>] ? xen_do_hypervisor_callback+0x1e/0x30
 <EOI>
 [<ffffffff810013aa>] ? hypercall_page+0x3aa/0x1000
 [<ffffffff810013aa>] ? hypercall_page+0x3aa/0x1000
 [<ffffffff8100768c>] ? xen_safe_halt+0xc/0x20
 [<ffffffff81013563>] ? default_idle+0x23/0x40
 [<ffffffff8100b073>] ? cpu_idle+0x63/0xb0
 [<ffffffff81654c43>] ? start_kernel+0x362/0x36d
 [<ffffffff81657491>] ? xen_start_kernel+0x558/0x55e
Code: 39 ed 0f 84 1c 02 00 00 44 8b 7b 48 4c 8b 73 50 41 83 ef 01 41
21 ef 49 6b c7 70 4d 8b 64 06 40 49 69 c4 d0 00 00 00 48 8d 14 03 <48>
8b 8a 78 02 00 00 48 89 4c 24 10 80 ba 09 02 00 00 00 74 6d
RIP  [<ffffffff8125ed66>] blkif_interrupt+0x66/0x320
 RSP <ffff88001fc03e18>
---[ end trace dfd4e5623eb06620 ]---

Regards,
-- 
William
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