nfs: use kmap/kunmap directly

This patch removes useless nfs_readdir_get_array() and
nfs_readdir_release_array() as suggested by Trond Myklebust

nfs_readdir() calls nfs_revalidate_mapping() before
readdir_search_pagecache() , nfs_do_filldir(), uncached_readdir()
so mapping should be correct.

While kmap() can't fail, all subsequent error checks were removed
as well as unused labels.

Signed-off-by: Fabian Frederick <fabf@skynet.be>
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>