hrtimer: fix issue during core isolation

During core isolation, while migrating hrtimers to one of online CPUs,
below issue is being observed due to commit 5da70160
("hrtimer: Implement support for softirq based hrtimers") which started
to enable local irqs. Fix it by moving local_bh_disable/enable calls to
hotplug path only.

[   73.488582] WARNING: CPU: 0 PID: 7 at kernel/softirq.c:106 \
					__local_bh_disable_ip+0xc4/0xd0
[   73.506182] WARNING: CPU: 0 PID: 7 at kernel/softirq.c:160 \
					__local_bh_enable_ip+0xd0/0x140
[   73.523834] kernel BUG at kernel/irq_work.c:138!
[   73.531979] Internal error: Oops - BUG: 0 [#1] PREEMPT SMP
[   73.567732] pstate: 60400005 (nZCv daif +PAN -UAO)
[   73.572663] pc : irq_work_run+0x160/0x170
[   73.576795] lr : flush_smp_call_function_queue+0x178/0x1b0
[   73.674966] Call trace:
[   73.677494]  irq_work_run+0x160/0x170
[   73.681271]  flush_smp_call_function_queue+0x178/0x1b0
[   73.686558]  generic_smp_call_function_single_interrupt+0x14/0x20
[   73.692832]  handle_IPI+0x188/0x2f8
[   73.696426]  gic_handle_irq+0x110/0x1d8
[   73.700374]  el1_irq+0xb4/0x130

Change-Id: I969171022377dc793834c4c4343e5f839f224dbe
Signed-off-by: Satya Durga Srinivasu Prabhala <satyap@codeaurora.org>