x86/apic: Make init_legacy_irqs() __init

This function is only called by arch_early_irq_init(), which is an
__init function, so mark the child function __init as well.

In addition mark it inline for the !CONFIG_X86_IO_APIC case.

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/1498040061-5332-1-git-send-email-douly.fnst@cn.fujitsu.com
Signed-off-by: Ingo Molnar <mingo@kernel.org>