rcu: add assertions to debug build

rcu: add assertions to debug build

Xen's RCU implementation relies on no softirq handling taking place
while being in a RCU critical section. Add ASSERT()s in debug builds
in order to catch any violations.

For that purpose modify rcu_read_[un]lock() to use a dedicated percpu
counter additional to preempt_[en|dis]able() as this enables to test
that condition in __do_softirq() (ASSERT_NOT_IN_ATOMIC() is not
usable there due to __cpu_up() calling process_pending_softirqs()
while holding the cpu hotplug lock).

While at it switch the rcu_read_[un]lock() implementation to static
inline functions instead of macros.

Signed-off-by: Juergen Gross <jgross@suse.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>