refscale: Add SRCU-fast-updown readers

This commit adds refscale readers based on srcu_read_lock_fast_updown()
and srcu_read_lock_fast_updown() ("refscale.scale_type=srcu-fast-updown").
On my x86 laptop, these are about 2.2ns per pair.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>
1 file changed