rcu/sync: Remove custom check for RCU readers

The rcu/sync code currently does a special check for being in an RCU
read-side critical section.  With RCU consolidating flavors and the
generic helper added earlier in this series, this check is no longer need.
This commit switches to the generic helper, saving a couple of lines
of code.

Cc: Oleg Nesterov <oleg@redhat.com>
Acked-by: Oleg Nesterov <oleg@redhat.com>
Signed-off-by: Joel Fernandes (Google) <joel@joelfernandes.org>
Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>