xen/smp/spinlock: Fix leakage of the spinlock interrupt line for every CPU online...

commit 66ff0fe9e7bda8aec99985b24daad03652f7304e upstream.

While we don't use the spinlock interrupt line (see for details
commit f10cd522c5fbfec9ae3cc01967868c9c2401ed23 -
xen: disable PV spinlocks on HVM) - we should still do the proper
init / deinit sequence. We did not do that correctly and for the
CPU init for PVHVM guest we would allocate an interrupt line - but
failed to deallocate the old interrupt line.

This resulted in leakage of an irq_desc but more importantly this splat
as we online an offlined CPU:

genirq: Flags mismatch irq 71. 0002cc20 (spinlock1) vs. 0002cc20 (spinlock1)
Pid: 2542, comm: init.late Not tainted 3.9.0-rc6upstream #1
Call Trace:
 [<ffffffff811156de>] __setup_irq+0x23e/0x4a0
 [<ffffffff81194191>] ? kmem_cache_alloc_trace+0x221/0x250
 [<ffffffff811161bb>] request_threaded_irq+0xfb/0x160
 [<ffffffff8104c6f0>] ? xen_spin_trylock+0x20/0x20
 [<ffffffff813a8423>] bind_ipi_to_irqhandler+0xa3/0x160
 [<ffffffff81303758>] ? kasprintf+0x38/0x40
 [<ffffffff8104c6f0>] ? xen_spin_trylock+0x20/0x20
 [<ffffffff810cad35>] ? update_max_interval+0x15/0x40
 [<ffffffff816605db>] xen_init_lock_cpu+0x3c/0x78
 [<ffffffff81660029>] xen_hvm_cpu_notify+0x29/0x33
 [<ffffffff81676bdd>] notifier_call_chain+0x4d/0x70
 [<ffffffff810bb2a9>] __raw_notifier_call_chain+0x9/0x10
 [<ffffffff8109402b>] __cpu_notify+0x1b/0x30
 [<ffffffff8166834a>] _cpu_up+0xa0/0x14b
 [<ffffffff816684ce>] cpu_up+0xd9/0xec
 [<ffffffff8165f754>] store_online+0x94/0xd0
 [<ffffffff8141d15b>] dev_attr_store+0x1b/0x20
 [<ffffffff81218f44>] sysfs_write_file+0xf4/0x170
 [<ffffffff811a2864>] vfs_write+0xb4/0x130
 [<ffffffff811a302a>] sys_write+0x5a/0xa0
 [<ffffffff8167ada9>] system_call_fastpath+0x16/0x1b
cpu 1 spinlock event irq -16
smpboot: Booting Node 0 Processor 1 APIC 0x2

And if one looks at the /proc/interrupts right after
offlining (CPU1):

  70:          0          0  xen-percpu-ipi       spinlock0
  71:          0          0  xen-percpu-ipi       spinlock1
  77:          0          0  xen-percpu-ipi       spinlock2

There is the oddity of the 'spinlock1' still being present.

Signed-off-by: Konrad Rzeszutek Wilk <konrad.wilk@oracle.com>
[bwh: Backported to 3.2: adjust context]
Signed-off-by: Ben Hutchings <ben@decadent.org.uk>