In kernels built with CONFIG_PREEMPT_RT=y, spinlocks are replaced by
rt_mutex, which can sleep.  This means that acquiring a non-raw spinlock
in a critical section where preemption is disabled can trigger the
following BUG:

BUG: scheduling while atomic: ref_scale_reade/76/0x00000002
Preemption disabled at:
ref_lock_section+0x16/0x80
Call Trace:
<TASK>
dump_stack_lvl+0x5b/0x82
dump_stack+0x10/0x12
__schedule_bug.cold+0x9c/0xad
__schedule+0x839/0xc00
schedule_rtlock+0x22/0x40
rtlock_slowlock_locked+0x460/0x1350
rt_spin_lock+0x61/0xe0
ref_lock_section+0x29/0x80
rcu_scale_one_reader+0x52/0x60
ref_scale_reader+0x28d/0x490
kthread+0x128/0x150
ret_from_fork+0x22/0x30
</TASK>

This commit therefore converts spinlock to raw_spinlock.
Signed-off-by: Zqiang <qiang1.zhang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>