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