lockdep: fix ftrace irq tracing false positive

fix this false positive:

[    0.020000] ------------[ cut here ]------------
[    0.020000] WARNING: at kernel/lockdep.c:2718 check_flags+0x14a/0x170()
[    0.020000] Modules linked in:
[    0.020000] Pid: 0, comm: swapper Not tainted 2.6.26-tip-00343-gd7e5521-dirty #14486
[    0.020000]  [<c01312e4>] warn_on_slowpath+0x54/0x80
[    0.020000]  [<c067e451>] ? _spin_unlock_irqrestore+0x61/0x70
[    0.020000]  [<c0131bb1>] ? release_console_sem+0x201/0x210
[    0.020000]  [<c0143d65>] ? __kernel_text_address+0x35/0x40
[    0.020000]  [<c010562e>] ? dump_trace+0x5e/0x140
[    0.020000]  [<c01518b5>] ? __lock_acquire+0x245/0x820
[    0.020000]  [<c015063a>] check_flags+0x14a/0x170
[    0.020000]  [<c0151ed8>] ? lock_acquire+0x48/0xc0
[    0.020000]  [<c0151ee1>] lock_acquire+0x51/0xc0
[    0.020000]  [<c014a16c>] ? down+0x2c/0x40
[    0.020000]  [<c010a609>] ? sched_clock+0x9/0x10
[    0.020000]  [<c067e7b2>] _write_lock+0x32/0x60
[    0.020000]  [<c013797f>] ? request_resource+0x1f/0xb0
[    0.020000]  [<c013797f>] request_resource+0x1f/0xb0
[    0.020000]  [<c02f89ad>] vgacon_startup+0x2bd/0x3e0
[    0.020000]  [<c094d62a>] con_init+0x19/0x22f
[    0.020000]  [<c0330c7c>] ? tty_register_ldisc+0x5c/0x70
[    0.020000]  [<c094cf49>] console_init+0x20/0x2e
[    0.020000]  [<c092a969>] start_kernel+0x20c/0x379
[    0.020000]  [<c092a516>] ? unknown_bootoption+0x0/0x1f6
[    0.020000]  [<c092a099>] __init_begin+0x99/0xa1
[    0.020000]  =======================
[    0.020000] ---[ end trace 4eaa2a86a8e2da22 ]---
[    0.020000] possible reason: unannotated irqs-on.
[    0.020000] irq event stamp: 0

which occurs if CONFIG_TRACE_IRQFLAGS=y, CONFIG_DEBUG_LOCKDEP=y,
but CONFIG_PROVE_LOCKING is disabled.

Signed-off-by: Ingo Molnar <mingo@elte.hu>