tipc: delete the unlikely branch in tipc_aead_encrypt

tipc: delete the unlikely branch in tipc_aead_encrypt

When a skb comes to tipc_aead_encrypt(), it's always linear. The
unlikely check 'skb_cloned(skb) && tailen <= skb_tailroom(skb)'
can completely be taken care of in skb_cow_data() by the code
in branch "if (!skb_has_frag_list())".

Also, remove the 'TODO:' annotation, as the pages in skbs are not
writable, see more on commit 6d604faa6f41 ("tipc: do not write
skb_shinfo frags when doing decrytion").

Signed-off-by: Xin Long <lucien.xin@gmail.com>
Acked-by: Jon Maloy <jmaloy@redhat.com>
Link: https://lore.kernel.org/r/47a478da0b6095b76e3cbe7a75cbd25d9da1df9a.1637773872.git.lucien.xin@gmail.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>