xentrace: reduce size of extradata in trace_irq_mask()

xentrace: reduce size of extradata in trace_irq_mask()

Reduce size of extra_data in to avoid possible crash in trace_var.

(XEN) Assertion 'extra_word <= TRACE_EXTRA_MAX' failed at trace.c:687
(XEN) Xen call trace:
(XEN)    [<ffff82c480128783>] __trace_var+0x4d/0x3b8
(XEN)    [<ffff82c480162172>] trace_irq_mask+0x49/0x4b
(XEN)    [<ffff82c4801631ae>] __assign_irq_vector+0x241/0x374
(XEN)    [<ffff82c48015d813>] set_desc_affinity+0x5d/0xd4
(XEN)    [<ffff82c480160708>] set_msi_affinity+0x44/0x1c1
(XEN)    [<ffff82c480162938>] move_masked_irq+0x9c/0xcd
(XEN)    [<ffff82c4801629a7>] move_native_irq+0x3e/0x53
(XEN)    [<ffff82c48015d969>] ack_msi_irq+0x2c/0x6e
(XEN)    [<ffff82c4801622e3>] do_IRQ+0x16f/0x66d

Signed-off-by: Olaf Hering <olaf@aepfle.de>
Acked-by: George Dunlap <george.dunlap@eu.citrix.com>