ZTS: user_namespace_004.ksh avoid error in cleanup if unsupported

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Seth Troisi <sethtroisi@google.com>
Closes #16114

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"