x86/traps: Document do_spurious_interrupt_bug()

Add a comment which explains why this empty handler for a reserved vector
exists.

Requested-by: Josh Poimboeuf <jpoimboe@redhat.com>

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Reviewed-by: Alexandre Chartre <alexandre.chartre@oracle.com>
Reviewed-by: Andy Lutomirski <luto@kernel.org>
Link: https://lkml.kernel.org/r/20200225220216.624165786@linutronix.de