dlm: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage

When transmitting data, call down a layer using a single sendmsg with
MSG_SPLICE_PAGES to indicate that content should be spliced rather using
sendpage.  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: Christine Caulfield <ccaulfie@redhat.com>
cc: David Teigland <teigland@redhat.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: cluster-devel@redhat.com
Link: https://lore.kernel.org/r/20230623225513.2732256-7-dhowells@redhat.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>