NFS: More readdir cleanups

Remove the redundant caching of the credential in struct
nfs_open_dir_context.
Pass the buffer size as an argument to nfs_readdir_xdr_filler().

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>
Reviewed-by: Benjamin Coddington <bcodding@redhat.com>
Tested-by: Benjamin Coddington <bcodding@redhat.com>
Tested-by: Dave Wysochanski <dwysocha@redhat.com>