torture: Save "make allmodconfig" .config file

Currently, torture.sh saves only the build output and exit code from the
"make allmodconfig" test.  This commit also saves the .config file.
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

tools/testing/selftests/rcutorture/bin/torture.sh

@@ -322,6 +322,7 @@ then
 	echo " --- make clean" > "$amcdir/Make.out" 2>&1
 	make -j$MAKE_ALLOTED_CPUS clean >> "$amcdir/Make.out" 2>&1
 	echo " --- make allmodconfig" >> "$amcdir/Make.out" 2>&1
+	cp .config $amcdir
 	make -j$MAKE_ALLOTED_CPUS allmodconfig >> "$amcdir/Make.out" 2>&1
 	echo " --- make " >> "$amcdir/Make.out" 2>&1
 	make -j$MAKE_ALLOTED_CPUS >> "$amcdir/Make.out" 2>&1