SUNRPC: Fix NFS READs that start at non-page-aligned offsets

Anj Duvnjak reports that the Kodi.tv NFS client is not able to read
video files from a v5.10.11 Linux NFS server.

The new sendpage-based TCP sendto logic was not attentive to non-
zero page_base values. nfsd_splice_read() sets that field when a
READ payload starts in the middle of a page.

The Linux NFS client rarely emits an NFS READ that is not page-
aligned. All of my testing so far has been with Linux clients, so I
missed this one.
Reported-by: A. Duvnjak <avian@extremenerds.net>
BugLink: https://bugzilla.kernel.org/show_bug.cgi?id=211471
Fixes: 4a85a6a3 ("SUNRPC: Handle TCP socket sends with kernel_sendpage() again")
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Tested-by: A. Duvnjak <avian@extremenerds.net>

net/sunrpc/svcsock.c

@@ -1113,14 +1113,15 @@ static int svc_tcp_sendmsg(struct socket *sock, struct msghdr *msg,
 		unsigned int offset, len, remaining;
 		struct bio_vec *bvec;
 
-		bvec = xdr->bvec;
-		offset = xdr->page_base;
+		bvec = xdr->bvec + (xdr->page_base >> PAGE_SHIFT);
+		offset = offset_in_page(xdr->page_base);
 		remaining = xdr->page_len;
 		flags = MSG_MORE | MSG_SENDPAGE_NOTLAST;
 		while (remaining > 0) {
 			if (remaining <= PAGE_SIZE && tail->iov_len == 0)
 				flags = 0;
-			len = min(remaining, bvec->bv_len);
+
+			len = min(remaining, bvec->bv_len - offset);
 			ret = kernel_sendpage(sock, bvec->bv_page,
 					      bvec->bv_offset + offset,
 					      len, flags);