SUNRPC: Fix the recent bv_offset fix

Jeff confirmed his original fix addressed his pynfs test failure,
but this same bug also impacted qemu: accessing qcow2 virtual disks
using direct I/O was failing. Jeff's fix missed that you have to
shorten the bio_vec element by the same amount as you increased
the page offset.
Reported-by: Maxim Levitsky <mlevitsk@redhat.com>
Fixes: c96e2a69 ("sunrpc: set the bv_offset of first bvec in svc_tcp_sendmsg")
Tested-by: Maxim Levitsky <mlevitsk@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>

net/sunrpc/svcsock.c

@@ -1244,8 +1244,10 @@ static int svc_tcp_sendmsg(struct socket *sock, struct xdr_buf *xdr,
 	if (ret != head->iov_len)
 		goto out;
 
-	if (xdr_buf_pagecount(xdr))
+	if (xdr_buf_pagecount(xdr)) {
 		xdr->bvec[0].bv_offset = offset_in_page(xdr->page_base);
+		xdr->bvec[0].bv_len -= offset_in_page(xdr->page_base);
+	}
 
 	msg.msg_flags = MSG_SPLICE_PAGES;
 	iov_iter_bvec(&msg.msg_iter, ITER_SOURCE, xdr->bvec,