RCU priority boosting currently is not applied until the grace period
is at least 250 milliseconds old (or the number of milliseconds specified
by the CONFIG_RCU_BOOST_DELAY Kconfig option).  Although this has worked
well, it can result in OOM under conditions of RCU callback flooding.
One can argue that the real-time systems using RCU priority boosting
should carefully avoid RCU callback flooding, but one can just as well
argue that an OOM is a rather obnoxious error message.

This commit therefore disables the RCU priority boosting delay when
there are excessive numbers of callbacks queued.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>