Don't use fsp_get_io_fd() to check if it's a pathref fd to refuse to grant a durable handle.

Currently we don't grant durable handle requests on opens for meta-data only (what we used to call "stat" opens, where we only have an O_PATH open fd). To refuse, we should check the fsp_flags.is_pathref flag directly, not call (fsp_get_io_fd(fsp) == -1). fsp_get_io_fd() will emit a scary error message if it wasn't a pathref fd and this isn't a fatal error, we're just using it to deny a durable handle open.

BUG: https://bugzilla.samba.org/show_bug.cgi?id=15042

Fix confirmed by submitter. Passes ci.