fuse integration: require an external mount to be a FUSE filesystem

A device-ID boundary alone does not prove FUSE: a tmpfs mount would pass mountedAt and report its results as FUSE compliance. Check the filesystem type (statfs magic on linux, fstypename on the BSDs); platforms without a check keep the mount and write verification.

test/fuse_integration/fuse_mount_linux.go

@@ -0,0 +1,17 @@
+//go:build linux
+
+package fuse
+
+import "syscall"
+
+// fuseSuperMagic is statfs f_type for fuse and fuseblk mounts.
+const fuseSuperMagic = 0x65735546
+
+// fuseFilesystem reports whether path sits on a FUSE mount.
+func fuseFilesystem(path string) bool {
+	var st syscall.Statfs_t
+	if err := syscall.Statfs(path, &st); err != nil {
+		return false
+	}
+	return st.Type == fuseSuperMagic
+}