srcu: Provide internal interface to start a Tiny SRCU grace period

srcu: Provide internal interface to start a Tiny SRCU grace period

There is a need for a polling interface for SRCU grace periods.
This polling needs to initiate an SRCU grace period without
having to queue (and manage) a callback.  This commit therefore
splits the Tiny SRCU call_srcu() function into callback-queuing and
start-grace-period portions, with the latter in a new function named
srcu_gp_start_if_needed().

Link: https://lore.kernel.org/rcu/20201112201547.GF3365678@moria.home.lan/
Reported-by: Kent Overstreet <kent.overstreet@gmail.com>
Reviewed-by: Neeraj Upadhyay <neeraju@codeaurora.org>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>