x86/apic: Split out spurious handling code

sysvec_spurious_apic_interrupt() calls into the handling body of
__spurious_interrupt() which is not obvious as that function is declared
inside the DEFINE_IDTENTRY_IRQ(spurious_interrupt) macro.

As __spurious_interrupt() is currently always inlined this ends up with two
copies of the same code for no reason.

Split the handling function out and invoke it from both entry points.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20210210002512.469379641@linutronix.de