rcu/nocb: Add Safe checks for access offloaded rdp

For built with CONFIG_PROVE_RCU=y and CONFIG_PREEMPT_RT=y kernels,
Disable BH does not change the SOFTIRQ corresponding bits in
preempt_count(), but change current->softirq_disable_cnt, this
resulted in the following splat:

WARNING: suspicious RCU usage
kernel/rcu/tree_plugin.h:36 Unsafe read of RCU_NOCB offloaded state!
stack backtrace:
CPU: 0 UID: 0 PID: 22 Comm: rcuc/0
Call Trace:
[    0.407907]  <TASK>
[    0.407910]  dump_stack_lvl+0xbb/0xd0
[    0.407917]  dump_stack+0x14/0x20
[    0.407920]  lockdep_rcu_suspicious+0x133/0x210
[    0.407932]  rcu_rdp_is_offloaded+0x1c3/0x270
[    0.407939]  rcu_core+0x471/0x900
[    0.407942]  ? lockdep_hardirqs_on+0xd5/0x160
[    0.407954]  rcu_cpu_kthread+0x25f/0x870
[    0.407959]  ? __pfx_rcu_cpu_kthread+0x10/0x10
[    0.407966]  smpboot_thread_fn+0x34c/0xa50
[    0.407970]  ? trace_preempt_on+0x54/0x120
[    0.407977]  ? __pfx_smpboot_thread_fn+0x10/0x10
[    0.407982]  kthread+0x40e/0x840
[    0.407990]  ? __pfx_kthread+0x10/0x10
[    0.407994]  ? rt_spin_unlock+0x4e/0xb0
[    0.407997]  ? rt_spin_unlock+0x4e/0xb0
[    0.408000]  ? __pfx_kthread+0x10/0x10
[    0.408006]  ? __pfx_kthread+0x10/0x10
[    0.408011]  ret_from_fork+0x40/0x70
[    0.408013]  ? __pfx_kthread+0x10/0x10
[    0.408018]  ret_from_fork_asm+0x1a/0x30
[    0.408042]  </TASK>

Currently, triggering an rdp offloaded state change need the
corresponding rdp's CPU goes offline, and at this time the rcuc
kthreads has already in parking state. this means the corresponding
rcuc kthreads can safely read offloaded state of rdp while it's
corresponding cpu is online.

This commit therefore add softirq_count() check for
Preempt-RT kernels.

Suggested-by: Joel Fernandes <joelagnelf@nvidia.com>
Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Zqiang <qiang.zhang1211@gmail.com>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>