selftests: forwarding: Redefine relative_path variable

The following code which is part of lib.sh:
relative_path="${BASH_SOURCE%/*}"
if [[ "$relative_path" == "${BASH_SOURCE}" ]]; then
       relative_path="."
fi

reimplements functionality that is part of `dirname`:
$ dirname ""
.

To avoid this duplication, replace "relative_path" by "net_forwarding_dir",
a new variable defined using dirname.

Furthermore, to avoid the potential confusion about what "relative_path" is
about (cwd, test script directory or test library directory), define
"net_forwarding_dir" as the absolute path to net/forwarding/.
Tested-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Hangbin Liu <liuhangbin@gmail.com>
Signed-off-by: Benjamin Poirier <bpoirier@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

tools/testing/selftests/net/forwarding/lib.sh

@@ -29,13 +29,10 @@ STABLE_MAC_ADDRS=${STABLE_MAC_ADDRS:=no}
 TCPDUMP_EXTRA_FLAGS=${TCPDUMP_EXTRA_FLAGS:=}
 TROUTE6=${TROUTE6:=traceroute6}
 
-relative_path="${BASH_SOURCE%/*}"
-if [[ "$relative_path" == "${BASH_SOURCE}" ]]; then
-	relative_path="."
-fi
+net_forwarding_dir=$(dirname "$(readlink -e "${BASH_SOURCE[0]}")")
 
-if [[ -f $relative_path/forwarding.config ]]; then
-	source "$relative_path/forwarding.config"
+if [[ -f $net_forwarding_dir/forwarding.config ]]; then
+	source "$net_forwarding_dir/forwarding.config"
 fi
 
 # Kselftest framework requirement - SKIP code is 4.

tools/testing/selftests/net/forwarding/mirror_gre_lib.sh

 # SPDX-License-Identifier: GPL-2.0
 
-source "$relative_path/mirror_lib.sh"
+source "$net_forwarding_dir/mirror_lib.sh"
 
 quick_test_span_gre_dir_ips()
 {

tools/testing/selftests/net/forwarding/mirror_gre_topo_lib.sh

@@ -33,7 +33,7 @@
 #   |                                                                         |
 #   +-------------------------------------------------------------------------+
 
-source "$relative_path/mirror_topo_lib.sh"
+source "$net_forwarding_dir/mirror_topo_lib.sh"
 
 mirror_gre_topo_h3_create()
 {