The device exported by smbd is for non-root user. smbd is running on a booster with only posix translator. posix_stat involves storing the inode generation number as an extended attribute. Since setattr is not permitted for non-root user, the operation fails.
Can you check if replacing this line: posix.c:250 ret = snprintf (gen_key, 1024, "trusted.%s.gen", this->name); with posix.c:250 ret = snprintf (gen_key, 1024, "user.%s.gen", this->name); fixes the problem? You'll also need to mount the backend filesystem with the "user_xattr" option. If this works, we can think about making "user." the default namespace for the generation number.
Although this problem could be fixed by changing the namespace to "user" from "trusted", doing so would pollute the user's xattr namespace. As this problem will only happen in the narrow case of libglusterfsclient+posix, marking as WONTFIX for now.