Make Chelsio's TLS offload sendmsg() support MSG_SPLICE_PAGES, splicing in
pages from the source iterator if possible and copying the data in
otherwise.

This allows ->sendpage() to be replaced by something that can handle
multiple multipage folios in a single transaction.
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Ayush Sawal <ayush.sawal@chelsio.com>
cc: "David S. Miller" <davem@davemloft.net>
cc: Eric Dumazet <edumazet@google.com>
cc: Jakub Kicinski <kuba@kernel.org>
cc: Paolo Abeni <pabeni@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: netdev@vger.kernel.org
Signed-off-by: Paolo Abeni <pabeni@redhat.com>