rcutorture: Make rcutorture_one_extend() check reader state

rcutorture: Make rcutorture_one_extend() check reader state

This commit adds reader-state debugging checks to a new function named
rcutorture_one_extend_check(), which is invoked before and after setting
new reader states by the existing rcutorture_one_extend() function.
These checks have proven to be rather heavyweight, reducing reproduction
rate of some failures by a factor of two.  They are therefore hidden
behind a new RCU_TORTURE_TEST_CHK_RDR_STATE Kconfig option.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Frederic Weisbecker <frederic@kernel.org>
Tested-by: kernel test robot <oliver.sang@intel.com>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>