rcu: Add fastpath bypassing funnel locking

In the common case, there will be only one expedited grace period in
the system at a given time, in which case it is not helpful to use
funnel locking.  This commit therefore adds a fastpath that bypasses
funnel locking when the root ->exp_funnel_mutex is not held.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

diff --cc Documentation/RCU/trace.txt
Simple merge

diff --cc kernel/rcu/tree.c
Simple merge

diff --cc kernel/rcu/tree.h
Simple merge

diff --cc kernel/rcu/tree_trace.c
Simple merge