selftests/bpf: Make test_doc_build.sh work from script directory

Previously, it fails as below:
-------------
root@lkp-skl-d01 /opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf# ./test_doc_build.sh
++ realpath --relative-to=/opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf ./test_doc_build.sh
+ SCRIPT_REL_PATH=test_doc_build.sh
++ dirname test_doc_build.sh
+ SCRIPT_REL_DIR=.
++ realpath /opt/rootfs/v5.14-rc4/tools/testing/selftests/bpf/./../../../../
+ KDIR_ROOT_DIR=/opt/rootfs/v5.14-rc4
+ cd /opt/rootfs/v5.14-rc4
+ for tgt in docs docs-clean
+ make -s -C /opt/rootfs/v5.14-rc4/. docs
make: *** No rule to make target 'docs'.  Stop.
+ for tgt in docs docs-clean
+ make -s -C /opt/rootfs/v5.14-rc4/. docs-clean
make: *** No rule to make target 'docs-clean'.  Stop.
-----------
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Li Zhijian <lizhijian@cn.fujitsu.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Song Liu <songliubraving@fb.com>
Link: https://lore.kernel.org/bpf/20210820015556.23276-3-lizhijian@cn.fujitsu.com

tools/testing/selftests/bpf/test_doc_build.sh

@@ -4,9 +4,10 @@ set -e
 
 # Assume script is located under tools/testing/selftests/bpf/. We want to start
 # build attempts from the top of kernel repository.
-SCRIPT_REL_PATH=$(realpath --relative-to=$PWD $0)
+SCRIPT_REL_PATH=$(realpath $0)
 SCRIPT_REL_DIR=$(dirname $SCRIPT_REL_PATH)
-KDIR_ROOT_DIR=$(realpath $PWD/$SCRIPT_REL_DIR/../../../../)
+KDIR_ROOT_DIR=$(realpath $SCRIPT_REL_DIR/../../../../)
+SCRIPT_REL_DIR=$(dirname $(realpath --relative-to=$KDIR_ROOT_DIR $SCRIPT_REL_PATH))
 cd $KDIR_ROOT_DIR
 
 for tgt in docs docs-clean; do