ceph: use osd_req_op_extent_osd_iter for netfs reads

The netfs layer has already pinned the pages involved before calling
issue_op, so we can just pass down the iter directly instead of calling
iov_iter_get_pages_alloc.

Instead of having to allocate a page array, use CEPH_MSG_DATA_ITER and
pass it the iov_iter directly to clone.

Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: Xiubo Li <xiubli@redhat.com>
Reviewed-and-tested-by: Luís Henriques <lhenriques@suse.de>
Reviewed-by: Milind Changire <mchangir@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>