fuse: TestDirectMount: ignore `Optional` field

As evidenced by the test failures that are reported, the
`Optional` may change value between mounts. Ignore it
in the comparison.

 --- FAIL: TestDirectMount (0.14s)
    mount_linux_test.go:173: DirectMount effective mount options mismatch:
        DirectMount: mountinfo.Info{ID:0, Parent:0, Major:0, Minor:0, Root:"", Mountpoint:"", Options:"rw,nosuid,nodev,relatime", Optional:"shared:417", FSType:"fuse./tmp/go-build3776165", Source:"a,b", VFSOptions:"rw,user_id=1000,group_id=1000,max_read=131072"}
        fusermount:  mountinfo.Info{ID:0, Parent:0, Major:0, Minor:0, Root:"", Mountpoint:"", Options:"rw,nosuid,nodev,relatime", Optional:"shared:373", FSType:"fuse./tmp/go-build3776165", Source:"a,b", VFSOptions:"rw,user_id=1000,group_id=1000,max_read=131072"}

Fixes https://github.com/hanwen/go-fuse/issues/481

Change-Id: Ie8105cf4bec7e7f461267217860cbf36e47ed329

fuse/mount_linux_test.go

@@ -138,7 +138,6 @@ func mountCheckOptions(t *testing.T, opts MountOptions) (info mountinfo.Info) {
 		Source:     orig.Source,
 		FSType:     orig.FSType,
 		VFSOptions: orig.VFSOptions,
-		Optional:   orig.Optional,
 	}
 	// server needs to run for Unmount to work
 	go srv.Serve()