cifs: Reapply lost fix from commit 30b2b219

Reapply the fix from:

   30b2b219 ("cifs: do not include page data when checking signature")

that got lost in the iteratorisation of the cifs driver.

Fixes: d08089f6 ("cifs: Change the I/O paths to use an iterator rather than a page list")
Acked-by: Paulo Alcantara (SUSE) <pc@manguebit.com>
Reported-by: Paulo Alcantara <pc@manguebit.com>
Signed-off-by: David Howells <dhowells@redhat.com>
cc: Paulo Alcantara <pc@cjr.nz>
cc: Shyam Prasad N <nspmangalore@gmail.com>
cc: Bharath S M <bharathsm@microsoft.com>
cc: Enzo Matsumiya <ematsumiya@suse.de>
cc: linux-cifs@vger.kernel.org
Signed-off-by: Steve French <stfrench@microsoft.com>

fs/cifs/smb2pdu.c

@@ -4180,10 +4180,12 @@ smb2_readv_callback(struct mid_q_entry *mid)
 	struct smb2_hdr *shdr =
 				(struct smb2_hdr *)rdata->iov[0].iov_base;
 	struct cifs_credits credits = { .value = 0, .instance = 0 };
-	struct smb_rqst rqst = { .rq_iov = &rdata->iov[1],
-				 .rq_nvec = 1,
-				 .rq_iter = rdata->iter,
-				 .rq_iter_size = iov_iter_count(&rdata->iter), };
+	struct smb_rqst rqst = { .rq_iov = &rdata->iov[1], .rq_nvec = 1 };
+
+	if (rdata->got_bytes) {
+		rqst.rq_iter	  = rdata->iter;
+		rqst.rq_iter_size = iov_iter_count(&rdata->iter);
+	};
 
 	WARN_ONCE(rdata->server != mid->server,
 		  "rdata server %p != mid server %p",