NFS: Reduce lock contention in nfs_page_find_head_request()

Add a lockless check for whether or not the page might be carrying
an existing writeback before we grab the inode->i_lock.
Reported-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>

fs/nfs/write.c

@@ -190,9 +190,11 @@ static struct nfs_page *nfs_page_find_head_request(struct page *page)
 	struct inode *inode = page_file_mapping(page)->host;
 	struct nfs_page *req = NULL;
 
-	spin_lock(&inode->i_lock);
-	req = nfs_page_find_head_request_locked(NFS_I(inode), page);
-	spin_unlock(&inode->i_lock);
+	if (PagePrivate(page) || PageSwapCache(page)) {
+		spin_lock(&inode->i_lock);
+		req = nfs_page_find_head_request_locked(NFS_I(inode), page);
+		spin_unlock(&inode->i_lock);
+	}
 	return req;
 }