scsi: target: iscsi: Use sendmsg(MSG_SPLICE_PAGES) rather than sendpage

Use sendmsg() with MSG_SPLICE_PAGES rather than sendpage.  This allows
multiple pages and multipage folios to be passed through.

TODO: iscsit_fe_sendpage_sg() should perhaps set up a bio_vec array for the
entire set of pages it's going to transfer plus two for the header and
trailer and page fragments to hold the header and trailer - and then call
sendmsg once for the entire message.

Signed-off-by: David Howells <dhowells@redhat.com>
cc: Mike Christie <michael.christie@oracle.com>
cc: Maurizio Lombardi <mlombard@redhat.com>
cc: "James E.J. Bottomley" <jejb@linux.ibm.com>
cc: "Martin K. Petersen" <martin.petersen@oracle.com>
cc: Jens Axboe <axboe@kernel.dk>
cc: Matthew Wilcox <willy@infradead.org>
cc: Al Viro <viro@zeniv.linux.org.uk>
cc: open-iscsi@googlegroups.com
Link: https://lore.kernel.org/r/20230623225513.2732256-13-dhowells@redhat.com


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