splice, net: Use sendmsg(MSG_SPLICE_PAGES) rather than ->sendpage()

Use sendmsg(MSG_SPLICE_PAGES) rather than ->sendpage() to splice data from
a pipe to a socket.  This paves the way for passing in multiple pages at
once from a pipe and the handling of multipage folios.

Signed-off-by: David Howells <dhowells@redhat.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