rds: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage

When transmitting data, call down into TCP using a single sendmsg with
MSG_SPLICE_PAGES to indicate that content should be spliced.

To make this work, the data is assembled in a bio_vec array and attached to
a BVEC-type iterator.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Santosh Shilimkar <santosh.shilimkar@oracle.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: rds-devel@oss.oracle.com
Link: https://lore.kernel.org/r/20230623225513.2732256-6-dhowells@redhat.com


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