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_pool.c

@@ -1200,7 +1200,7 @@ dsl_pool_user_hold_rele_impl(dsl_pool_t *dp, uint64_t dsobj,
 		error = zap_add(mos, zapobj, name, 8, 1, &now, tx);
 	else
 		error = zap_remove(mos, zapobj, name, tx);
-	strfree(name);
+	kmem_strfree(name);
 
 	return (error);
 }