cifs: Increase size of tmp_buf in cifs_readdir to avoid potential overflows

Increase size of tmp_buf to possible maximum to avoid potential
overflows.

Pointed-out-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Suresh Jayaraman <sjayaraman@suse.de>
Acked-by: Jeff Layton <jlayton@redhat.com>
Signed-off-by: Steve French <sfrench@us.ibm.com>