rcu-tasks: Fix code-style issues

This commit declares trc_n_readers_need_end and trc_wait static and
replaced a "&" with "&&".  The "&" happened to work because the values
are bool, but accidents waiting to happen and all that...

Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>