MNT_BYFSID
if (unmount(fsidbuf, fflag | MNT_BYFSID) != 0) {
if (flags & MNT_BYFSID) {
MNT_FLAG(MNT_BYFSID);
MNT_BYFSID | MNT_NOCOVER | MNT_EMPTYDIR | \
flags = MNT_BYFSID;
error = unmount(fsid_str, MNT_NONBUSY | MNT_BYFSID);