Avoid error in cleanup if test if unsupported

Signed-off-by: Seth Troisi <sethtroisi@google.com>

tests/zfs-tests/tests/functional/user_namespace/user_namespace_004.ksh

@@ -44,8 +44,6 @@ user_ns_cleanup() {
 	log_must zfs destroy -r "$TESTPOOL/userns"
 }
 
-log_onexit user_ns_cleanup
-
 log_assert "Check zfs zone command handling of non-namespace files"
 
 # Pass if user namespaces are not supported.
@@ -54,6 +52,8 @@ if [ "$?" -ne "0" ]; then
 	log_unsupported "Failed to create user namespace"
 fi
 
+log_onexit user_ns_cleanup
+
 # Create the baseline datasets.
 log_must zfs create -o zoned=on "$TESTPOOL/userns"