rcutorture: Suppress "Writer stall state" reports during boot

rcutorture: Suppress "Writer stall state" reports during boot

When rcutorture is running on only the one boot-time CPU while that CPU
is busy invoking initcall() functions, the added load is quite likely to
unduly delay the RCU grace-period kthread, rcutorture readers, and much
else besides.  This can result in rcu_torture_stats_print() reporting
rcutorture writer stalls, which are not really a bug in that environment.
After all, one CPU can only do so much.

This commit therefore suppresses rcutorture writer stalls while the
kernel is booting, that is, while rcu_inkernel_boot_has_ended() continues
returning false.

Signed-off-by: Paul E. McKenney <paulmck@kernel.org>