selftests/bpf: Fix vmtest.sh -h to not require root

Set the exit trap only after argument parsing is done. This way argument
parse failure or `-h` will not require sudo.

Reasoning is that it's confusing that a help message would require root
access.

Signed-off-by: Daniel Xu <dxu@dxuuu.xyz>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Daniel Müller <deso@posteo.net>
Link: https://lore.kernel.org/bpf/6a802aa37758e5a7e6aa5de294634f5518005e2b.1660064925.git.dxu@dxuuu.xyz

tools/testing/selftests/bpf/vmtest.sh

@@ -307,6 +307,20 @@ update_kconfig()
 	fi
 }
 
+catch()
+{
+	local exit_code=$1
+	local exit_status_file="${OUTPUT_DIR}/${EXIT_STATUS_FILE}"
+	# This is just a cleanup and the directory may
+	# have already been unmounted. So, don't let this
+	# clobber the error code we intend to return.
+	unmount_image || true
+	if [[ -f "${exit_status_file}" ]]; then
+		exit_code="$(cat ${exit_status_file})"
+	fi
+	exit ${exit_code}
+}
+
 main()
 {
 	local script_dir="$(cd -P -- "$(dirname -- "${BASH_SOURCE[0]}")" && pwd -P)"
@@ -353,6 +367,8 @@ main()
 	done
 	shift $((OPTIND -1))
 
+	trap 'catch "$?"' EXIT
+
 	if [[ $# -eq 0  && "${debug_shell}" == "no" ]]; then
 		echo "No command specified, will run ${DEFAULT_COMMAND} in the vm"
 	else
@@ -409,20 +425,4 @@ main()
 	fi
 }
 
-catch()
-{
-	local exit_code=$1
-	local exit_status_file="${OUTPUT_DIR}/${EXIT_STATUS_FILE}"
-	# This is just a cleanup and the directory may
-	# have already been unmounted. So, don't let this
-	# clobber the error code we intend to return.
-	unmount_image || true
-	if [[ -f "${exit_status_file}" ]]; then
-		exit_code="$(cat ${exit_status_file})"
-	fi
-	exit ${exit_code}
-}
-
-trap 'catch "$?"' EXIT
-
 main "$@"