rcu/nocb: Extract nocb_bypass_needs_flush() to reduce duplication

rcu/nocb: Extract nocb_bypass_needs_flush() to reduce duplication

The bypass flush decision logic is duplicated in rcu_nocb_try_bypass()
and nocb_gp_wait() with similar conditions.

This commit therefore extracts the functionality into a common helper
function nocb_bypass_needs_flush() improving the code readability.

A flush_faster parameter is added to controlling the flushing thresholds
and timeouts. This design was in the original commit d1b222c6be1f
("rcu/nocb: Add bypass callback queueing") to avoid having the GP
kthread aggressively flush the bypass queue.

Reviewed-by: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Joel Fernandes <joelagnelf@nvidia.com>