x86/io-apic: prevent early exit from i8259 loop detection

x86/io-apic: prevent early exit from i8259 loop detection

Avoid exiting early from the loop when a pin that could be connected to the
i8259 is found, as such early exit would leave the EOI handler translation
array only partially allocated and/or initialized.

Otherwise on systems with multiple IO-APICs and an unmasked ExtINT pin on
any IO-APIC that's no the last one the following NULL pointer dereference
triggers:

(XEN) Enabling APIC mode.  Using 2 I/O APICs
(XEN) ----[ Xen-4.20-unstable  x86_64  debug=y  Not tainted ]----
(XEN) CPU:    0
(XEN) RIP:    e008:[<ffff82d040328046>] __ioapic_write_entry+0x83/0x95
[...]
(XEN) Xen call trace:
(XEN)    [<ffff82d040328046>] R __ioapic_write_entry+0x83/0x95
(XEN)    [<ffff82d04027464b>] F amd_iommu_ioapic_update_ire+0x1ea/0x273
(XEN)    [<ffff82d0402755a1>] F iommu_update_ire_from_apic+0xa/0xc
(XEN)    [<ffff82d040328056>] F __ioapic_write_entry+0x93/0x95
(XEN)    [<ffff82d0403283c1>] F arch/x86/io_apic.c#clear_IO_APIC_pin+0x7c/0x10e
(XEN)    [<ffff82d040328480>] F arch/x86/io_apic.c#clear_IO_APIC+0x2d/0x61
(XEN)    [<ffff82d0404448b7>] F enable_IO_APIC+0x2e3/0x34f
(XEN)    [<ffff82d04044c9b0>] F smp_prepare_cpus+0x254/0x27a
(XEN)    [<ffff82d04044bec2>] F __start_xen+0x1ce1/0x23ae
(XEN)    [<ffff82d0402033ae>] F __high_start+0x8e/0x90
(XEN)
(XEN) Pagetable walk from 0000000000000000:
(XEN)  L4[0x000] = 000000007dbfd063 ffffffffffffffff
(XEN)  L3[0x000] = 000000007dbfa063 ffffffffffffffff
(XEN)  L2[0x000] = 000000007dbcc063 ffffffffffffffff
(XEN)  L1[0x000] = 0000000000000000 ffffffffffffffff
(XEN)
(XEN) ****************************************
(XEN) Panic on CPU 0:
(XEN) FATAL PAGE FAULT
(XEN) [error_code=0002]
(XEN) Faulting linear address: 0000000000000000
(XEN) ****************************************
(XEN)
(XEN) Reboot in five seconds...

Reported-by: Sergii Dmytruk <sergii.dmytruk@3mdeb.com>
Fixes: 86001b3970fe ('x86/io-apic: fix directed EOI when using AMD-Vi interrupt remapping')
Signed-off-by: Roger Pau Monné <roger.pau@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>