x86/vector: Merge allocate_vector() into assign_vector_locked()

assign_vector_locked() calls allocate_vector() to get a real vector for an
IRQ. If the current target CPU is online and in the new requested affinity
mask, allocate_vector() will return 0 and nothing should be done. But,
assign_vector_locked() calls apic_update_irq_cfg() even in that case which
is pointless.

allocate_vector() is not called from anything else, so the functions can be
merged and in case of no change the apic_update_irq_cfg() can be avoided.

[ tglx: Massaged changelog ]

Signed-off-by: Dou Liyang <douly.fnst@cn.fujitsu.com>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: hpa@zytor.com
Link: https://lkml.kernel.org/r/20180511080956.6316-1-douly.fnst@cn.fujitsu.com