Use mp_ops->send_ipi_single() instead of mp_ops->send_ipi_mask() in
arch_send_call_function_single_ipi(). send_ipi_single() can send
IPI signal to a special cpu more efficiently.

Signed-off-by: Liangliang Huang <huangll@lemote.com>
Reviewed-by: Huacai Chen <chenhc@lemote.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>