rcu: Make force_quiescent_state() start grace period if needed

Grace periods cannot be started while force_quiescent_state() is
active.  This is OK in that the affected CPUs will try again
later, but it does induce needless grace-period delays.  This
patch causes rcu_start_gp() to record a failed attempt to start
a grace period. When force_quiescent_state() prepares to return,
it then starts the grace period if there was such a failed
attempt.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: laijs@cn.fujitsu.com
Cc: dipankar@in.ibm.com
Cc: mathieu.desnoyers@polymtl.ca
Cc: josh@joshtriplett.org
Cc: dvhltc@us.ibm.com
Cc: niv@us.ibm.com
Cc: peterz@infradead.org
Cc: rostedt@goodmis.org
Cc: Valdis.Kletnieks@vt.edu
Cc: dhowells@redhat.com
LKML-Reference: <12626465501854-git-send-email->
Signed-off-by: Ingo Molnar <mingo@elte.hu>