tcp_bpf: Make tcp_bpf_sendpage() go through tcp_bpf_sendmsg(MSG_SPLICE_PAGES)

Make tcp_bpf_sendpage() a wrapper around tcp_bpf_sendmsg(MSG_SPLICE_PAGES)
rather than a loop calling tcp_sendpage().  sendpage() will be removed in
the future.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: John Fastabend <john.fastabend@gmail.com>
cc: Jakub Sitnicki <jakub@cloudflare.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>