ceph: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage()

Use sendmsg() and MSG_SPLICE_PAGES rather than sendpage in ceph when
transmitting data.  For the moment, this can only transmit one page at a
time because of the architecture of net/ceph/, but if
write_partial_message_data() can be given a bvec[] at a time by the
iteration code, this would allow pages to be sent in a batch.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Ilya Dryomov <idryomov@gmail.com>
cc: Xiubo Li <xiubli@redhat.com>
cc: Jeff Layton <jlayton@kernel.org>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
Link: https://lore.kernel.org/r/20230623225513.27322563

-5-dhowells@redhat.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>