rcu: Add polled expedited grace-period primitives

This commit adds expedited grace-period functionality to RCU's polled
grace-period API, adding start_poll_synchronize_rcu_expedited() and
cond_synchronize_rcu_expedited(), which are similar to the existing
start_poll_synchronize_rcu() and cond_synchronize_rcu() functions,
respectively.

Note that although start_poll_synchronize_rcu_expedited() can be invoked
very early, the resulting expedited grace periods are not guaranteed
to start until after workqueues are fully initialized.  On the other
hand, both synchronize_rcu() and synchronize_rcu_expedited() can also
be invoked very early, and the resulting grace periods will be taken
into account as they occur.

[ paulmck: Apply feedback from Neeraj Upadhyay. ]

Link: https://lore.kernel.org/all/[email protected]/
Link: https://docs.google.com/document/d/1RNKWW9jQyfjxw2E8dsXVTdvZYh0HnYeSHDKog9jhdN8/edit?usp=sharing
Cc: Brian Foster <[email protected]>
Cc: Dave Chinner <[email protected]>
Cc: Al Viro <[email protected]>
Cc: Ian Kent <[email protected]>
Signed-off-by: Paul E. McKenney <[email protected]>

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