kernel - Try to ensure a uniform fsid for exports

kernel - Try to ensure a uniform fsid for exports

* For filesystems which do not generate their own fsid (hammer2, tmpfs,
  others) we generate a fsid based on the mount path.

* Ensure fsid is uniquified when inserting onto the mount list.
  Collisions should be rare but the hash is only around 24 bits.

* Index the mount structure on a RB tree via the fsid.