torture: Suppress propagating trace_printk() warning

When trace_printk() is used, a message including "BUG" is printed to
the console, which fools the rcutorture scripting into believing that
the corresponding test scenario failed.  This commit therefore filters
out this particular instance of "BUG", thus avoiding the false-positive
test-failure report.

Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>