selftests/bpf: Convert test_xdp_redirect.sh to bash

The test_xdp_redirect.sh script uses a bash feature, '&>'. On systems,
e.g. Debian, where '/bin/sh' is dash, this will not work as
expected. Use bash in the shebang to get the expected behavior.

Further, using 'set -e' means that the error of a command cannot be
captured without the command being executed with '&&' or '||'. Let us
restructure the ping-commands, and use them as an if-expression, so
that we can capture the return value.

v4: Added missing Fixes:, and removed local variables. (Andrii)
v3: Reintroduced /bin/bash, and kept 'set -e'. (Andrii)
v2: Kept /bin/sh and removed bashisms. (Randy)

Fixes: 996139e8 ("selftests: bpf: add a test for XDP redirect")
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20210211082029.1687666-1-bjorn.topel@gmail.com

tools/testing/selftests/bpf/test_xdp_redirect.sh

-#!/bin/sh
+#!/bin/bash
 # Create 2 namespaces with two veth peers, and
 # forward packets in-between using generic XDP
 #
@@ -57,12 +57,8 @@ test_xdp_redirect()
 	ip link set dev veth1 $xdpmode obj test_xdp_redirect.o sec redirect_to_222 &> /dev/null
 	ip link set dev veth2 $xdpmode obj test_xdp_redirect.o sec redirect_to_111 &> /dev/null
 
-	ip netns exec ns1 ping -c 1 10.1.1.22 &> /dev/null
-	local ret1=$?
-	ip netns exec ns2 ping -c 1 10.1.1.11 &> /dev/null
-	local ret2=$?
-
-	if [ $ret1 -eq 0 -a $ret2 -eq 0 ]; then
+	if ip netns exec ns1 ping -c 1 10.1.1.22 &> /dev/null &&
+	   ip netns exec ns2 ping -c 1 10.1.1.11 &> /dev/null; then
 		echo "selftests: test_xdp_redirect $xdpmode [PASS]";
 	else
 		ret=1