NFS: Avoid duplicate uncached readdir calls on eof

If we've reached the end of the directory, then cache that information
in the context so that we don't need to do an uncached readdir in order
to rediscover that fact.

Fixes: 794092c57f ("NFS: Do uncached readdir when we're seeking a cookie in an empty page cache")
Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>

include/linux/nfs_fs.h

@@ -107,6 +107,7 @@ struct nfs_open_dir_context {
 	__u64 dup_cookie;
 	pgoff_t page_index;
 	signed char duped;
+	bool eof;
 };
 
 /*