rcutorture: Add ability to test srcu_read_{,un}lock_fast()

This commit permits rcutorture to test srcu_read_{,un}lock_fast(), which
is specified by the rcutorture.reader_flavor=0x8 kernel boot parameter.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
Cc: Alexei Starovoitov <ast@kernel.org>
Cc: Andrii Nakryiko <andrii@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Kent Overstreet <kent.overstreet@linux.dev>
Cc: <bpf@vger.kernel.org>
Signed-off-by: Boqun Feng <boqun.feng@gmail.com>