x86/apic/ipi: Tidy up the code and fixup comments

Replace the undecodable comment on top of the function, replace the space
consuming zero content comments with useful ones and tidy up the
implementation to prevent further eye bleed.

Make __default_send_IPI_shortcut() static as it has no other users.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Dave Hansen <dave.hansen@linux.intel.com>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Michael Kelley <mikelley@microsoft.com>
Tested-by: Sohil Mehta <sohil.mehta@intel.com>
Tested-by: Juergen Gross <jgross@suse.com> # Xen PV (dom0 and unpriv. guest)