pNFS: Add tracking to limit the number of pNFS retries

When the client is reading or writing using pNFS, and hits an error
on the DS, then it typically sends a LAYOUTERROR and/or LAYOUTRETURN
to the MDS, before redirtying the failed pages, and going for a new
round of reads/writebacks. The problem is that if the server has no
way to fix the DS, then we may need a way to interrupt this loop
after a set number of attempts have been made.
This patch adds an optional module parameter that allows the admin
to specify how many times to retry the read/writeback process before
failing with a fatal error.
The default behaviour is to retry forever.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

fs/nfs/direct.c

@@ -663,6 +663,8 @@ static void nfs_direct_write_reschedule(struct nfs_direct_req *dreq)
 	}
 
 	list_for_each_entry_safe(req, tmp, &reqs, wb_list) {
+		/* Bump the transmission count */
+		req->wb_nio++;
 		if (!nfs_pageio_add_request(&desc, req)) {
 			nfs_list_move_request(req, &failed);
 			spin_lock(&cinfo.inode->i_lock);
@@ -703,6 +705,11 @@ static void nfs_direct_commit_complete(struct nfs_commit_data *data)
 		req = nfs_list_entry(data->pages.next);
 		nfs_list_remove_request(req);
 		if (dreq->flags == NFS_ODIRECT_RESCHED_WRITES) {
+			/*
+			 * Despite the reboot, the write was successful,
+			 * so reset wb_nio.
+			 */
+			req->wb_nio = 0;
 			/* Note the rewrite will go through mds */
 			nfs_mark_request_commit(req, NULL, &cinfo, 0);
 		} else