srcu: Add might_sleep() annotation to synchronize_srcu()

Although synchronize_srcu() can sleep, it will not sleep if the fast
path succeeds, which means that illegal use of synchronize_rcu()
might go unnoticed.  This commit therefore adds might_sleep(), which
unconditionally catches illegal use of synchronize_rcu() from atomic
context.

Signed-off-by: Lai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>