NFS: add readdir cache array

This patch adds the readdir cache array and functions to retreive the array
stored on a cache page, clear the array by freeing allocated memory, add an
entry to the array, and search the array for a given cookie.

It then modifies readdir to make use of the new cache array.
With the new cache array method, we no longer need some of this code.

Finally, nfs_llseek_dir() will set file->f_pos to a value greater than 0 and
desc->dir_cookie to zero.  When we see this, readdir needs to find the file
at position file->f_pos from the start of the directory.
Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>