srcu: Expedited grace periods with reduced memory contention

Commit f60d231a ("srcu: Crude control of expedited grace periods")
introduced a per-srcu_struct atomic counter to track outstanding
requests for grace periods.  This works, but represents a memory-contention
bottleneck.  This commit therefore uses the srcu_node combining tree
to remove this bottleneck.

This commit adds new ->srcu_gp_seq_needed_exp fields to the
srcu_data, srcu_node, and srcu_struct structures, which track the
farthest-in-the-future grace period that must be expedited, which in
turn requires that all nearer-term grace periods also be expedited.
Requests for expediting start with the srcu_data structure, run up
through the srcu_node tree, and end at the srcu_struct structure.
Note that it may be necessary to expedite a grace period that just
now started, and this is handled by a new srcu_funnel_exp_start()
function, which is invoked when the grace period itself is already
in its way, but when that grace period was not marked as expedited.

A new srcu_get_delay() function returns zero if there is at least one
expedited SRCU grace period in flight, or SRCU_INTERVAL otherwise.
This function is used to calculate delays:  Normal grace periods
are allowed to extend in order to cover more requests with a given
grace-period computation, which decreases per-request overhead.

Signed-off-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Tested-by: Mike Galbraith <efault@gmx.de>