svcrdma: preserve rq_next_page in svc_rdma_save_io_pages

svcrdma: preserve rq_next_page in svc_rdma_save_io_pages

svc_rdma_save_io_pages() transfers response pages to the send
context and sets those slots to NULL. It then resets rq_next_page to
equal rq_respages, hiding the NULL region from
svc_rqst_release_pages().

Now that svc_rqst_release_pages() handles NULL entries, this reset
is no longer necessary. Removing it preserves the invariant that the
range [rq_respages, rq_next_page) accurately describes how many
response pages were consumed, enabling a subsequent optimization in
svc_alloc_arg() that refills only the consumed range.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>