NFS: Fix an Oopsable condition in __nfs_pageio_add_request()

Ensure that nfs_pageio_error_cleanup() resets the mirror array contents,
so that the structure reflects the fact that it is now empty.
Also change the test in nfs_pageio_do_add_request() to be more robust by
checking whether or not the list is empty rather than relying on the
value of pg_count.

Fixes: a7d42ddb ("nfs: add mirroring support to pgio layer")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>

fs/nfs/pagelist.c

@@ -1094,15 +1094,16 @@ nfs_pageio_do_add_request(struct nfs_pageio_descriptor *desc,
 	struct nfs_page *prev = NULL;
 	unsigned int size;
 
-	if (mirror->pg_count != 0) {
-		prev = nfs_list_entry(mirror->pg_list.prev);
-	} else {
+	if (list_empty(&mirror->pg_list)) {
 		if (desc->pg_ops->pg_init)
 			desc->pg_ops->pg_init(desc, req);
 		if (desc->pg_error < 0)
 			return 0;
 		mirror->pg_base = req->wb_pgbase;
-	}
+		mirror->pg_count = 0;
+		mirror->pg_recoalesce = 0;
+	} else
+		prev = nfs_list_entry(mirror->pg_list.prev);
 
 	if (desc->pg_maxretrans && req->wb_nio > desc->pg_maxretrans) {
 		if (NFS_SERVER(desc->pg_inode)->flags & NFS_MOUNT_SOFTERR)