Ensure fdescfs is mounted on /dev/fd/ in the length testcase

contrib/netbsd-tests/bin/dd/t_dd.sh

@@ -44,6 +44,12 @@ length_head() {
 	                "the one expected to fail.  (NetBSD PR bin/8521)"
 }
 length_body() {
+	# Begin FreeBSD
+	if ! df /dev/fd | grep -q '^fdescfs'; then
+		atf_skip "fdescfs is not mounted on /dev/fd"
+	fi
+	# End FreeBSD
+
 	test_dd_length 512 \
 	    "dd if=/dev/zero of=/dev/fd/5 count=1 5>&1 >/dev/null 2>/dev/null"
 	test_dd_length 512 \