srcu: Remove checks preventing offline CPUs from calling srcu_read_lock()

srcu: Remove checks preventing offline CPUs from calling srcu_read_lock()

SRCU has its own statemachine and no longer relies on normal RCU.
Its read-side critical section can now be used by an offline CPU, so this
commit removes the check and the comments, reverting the SRCU portion
of c423c6d2 (rcu: Check for illegal use of RCU from offlined CPUs).

It also makes the code match the comments in whatisRCU.txt:

g.	Do you need read-side critical sections that are respected
	even though they are in the middle of the idle loop, during
	user-mode execution, or on an offlined CPU?  If so, SRCU is the
	only choice that will work for you.

[ paulmck: There is at least one remaining issue, namely use of lockdep
	   with tracing enabled. ]

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