The tt_internal is really just a list_head to manage registered target_type
in a double linked list,

Here embed the list_head into target_type directly,
1. to avoid kmalloc/kfree;
2. then tt_internal is really unneeded;

Cc: stable@kernel.org
Signed-off-by: NCheng Renquan <crquan@gmail.com>
Signed-off-by: NAlasdair G Kergon <agk@redhat.com>
Reviewed-by: NAlasdair G Kergon <agk@redhat.com>