rcutorture: Make kvm-recheck-rcu.sh handle truncated lines

System hangs or killed rcutorture guest OSes can result in truncated
"Reader Pipe:" lines, which can in turn result in false-positive
reader-batch near-miss warnings.  This commit therefore adjusts the
reader-batch checks to account for possible line truncation.

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