EXP rcu: Add polled expedited grace-period primitives

EXP rcu: Add polled expedited grace-period primitives

This is an experimental proof of concept of polled expedited grace-period
functions.  These functions are get_state_synchronize_rcu_expedited(),
start_poll_synchronize_rcu_expedited(), poll_state_synchronize_rcu_expedited(),
and cond_synchronize_rcu_expedited(), which are similar to
get_state_synchronize_rcu(), start_poll_synchronize_rcu(),
poll_state_synchronize_rcu(), and cond_synchronize_rcu(), respectively.

One limitation is that start_poll_synchronize_rcu_expedited() cannot
be invoked before workqueues are initialized.

[ paulmck: Apply feedback from Neeraj Upadhyay. ]

Link: https://lore.kernel.org/all/20220121142454.1994916-1-bfoster@redhat.com/
Link: https://docs.google.com/document/d/1RNKWW9jQyfjxw2E8dsXVTdvZYh0HnYeSHDKog9jhdN8/edit?usp=sharing
Cc: Brian Foster <bfoster@redhat.com>
Cc: Dave Chinner <david@fromorbit.com>
Cc: Al Viro <viro@zeniv.linux.org.uk>
Cc: Ian Kent <raven@themaw.net>
Tested-by: Oliver Sang <oliver.sang@intel.com>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>