sched/core: Disable irq before calling irq_migrate_all_off_this_cpu

irq_migrate_all_off_this_cpu is not disabling the irq
when when irqs are migrated.
commit 0dd945ff ("genirq/cpuhotplug: Remove irq disabling logic")
removed the irq disable, thinking that every caller of
irq_migrate_all_off_this_cpu would disable irqs. So disable irqs
during the core isolation before calling irq_migrate_all_off_this_cpu.

Change-Id: I07b42e4ab6ce488e41f181ecf3033de20da8f831
Signed-off-by: Prasad Sodagudi <psodagud@codeaurora.org>