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