rcutorture: Use symbols for SRCU reader flavors

This commit converts rcutorture.c values for the reader_flavor module
parameter from hexadecimal to the SRCU_READ_FLAVOR_* C-preprocessor
macros.  The actual modprobe or kernel-boot-parameter values for
read_flavor must still be entered in hexadecimal.

Link: https://lore.kernel.org/all/c48c9dca-fe07-4833-acaa-28c827e5a79e@amd.com/



Suggested-by: Neeraj Upadhyay <Neeraj.Upadhyay@amd.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Uladzislau Rezki (Sony) <urezki@gmail.com>