refscale: Add srcu_read_lock_fast() support using "srcu-fast"

This commit creates a new srcu-fast option for the refscale.scale_type
module parameter that selects srcu_read_lock_fast() and
srcu_read_unlock_fast().

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>