Introduces call_rcu_bh() to be used when critical sections are mostly in
softirq context.

This patch introduces a new api - call_rcu_bh().  This is to be used for RCU
callbacks for whom the critical sections are mostly in softirq context.  These
callbacks consider completion of a softirq handler to be a quiescent state.
So, in order to make reader critical sections safe in process context,
rcu_read_lock_bh() and rcu_read_unlock_bh() must be used.  Use of softirq
handler completion as a quiescent state speeds up RCU grace periods and
prevents too many callbacks getting queued up in softirq-heavy workloads like
network stack.
Signed-off-by: Dipankar Sarma <dipankar@in.ibm.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>