NFS: Convert readdir page cache to use a cookie based index

Instead of using a linear index to address the pages, use the cookie of
the first entry, since that is what we use to match the page anyway.

This allows us to avoid re-reading the entire cache on a seekdir() type
of operation. The latter is very common when re-exporting NFS, and is a
major performance drain.

The change does affect our duplicate cookie detection, since we can no
longer rely on the page index as a linear offset for detecting whether
we looped backwards. However since we no longer do a linear search
through all the pages on each call to nfs_readdir(), this is less of a
concern than it was previously.
The other downside is that invalidate_mapping_pages() no longer can use
the page index to avoid clearing pages that have been read. A subsequent
patch will restore the functionality this provides to the 'ls -l'
heuristic.

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

include/linux/nfs_fs.h

@@ -106,11 +106,9 @@ struct nfs_open_dir_context {
 	unsigned long attr_gencount;
 	__be32	verf[NFS_DIR_VERIFIER_SIZE];
 	__u64 dir_cookie;
-	__u64 dup_cookie;
 	__u64 last_cookie;
 	pgoff_t page_index;
 	unsigned int dtsize;
-	signed char duped;
 	bool eof;
 	struct rcu_head rcu_head;
 };