rcutorture: Complain when invalid SRCU reader_flavor is specified

Currently, rcutorture ignores reader_flavor bits that are not in the
SRCU_READ_FLAVOR_ALL bitmask, which could confuse rcutorture users into
believing buggy patches had been fully tested.  This commit therefore
produces a splat in this case.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>