Fix strdup conflict on other platforms

In the FreeBSD kernel the strdup signature is:

```
char	*strdup(const char *__restrict, struct malloc_type *);
```

It's unfortunate that the developers have chosen to change
the signature of libc functions - but it's what I have to
deal with.

Reviewed-by: Jorgen Lundman <lundman@lundman.net>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Matt Macy <mmacy@FreeBSD.org>
Closes #9433

module/zfs/dsl_userhold.c

@@ -406,7 +406,7 @@ dsl_dataset_user_release_check_one(dsl_dataset_user_release_arg_t *ddura,
 				    snapname, holdname);
 				fnvlist_add_int32(ddura->ddura_errlist, errtag,
 				    ENOENT);
-				strfree(errtag);
+				kmem_strfree(errtag);
 			}
 			continue;
 		}