delete_pipe uses wrong constant DN_NR_HASH_MAX(16) instead of DN_PIPE_NR_MAX(65536): pipes 17-65536 permanently undeletable

Summary

delete_pipe(:1654-1655) validates pipe_nr>DN_NR_HASH_MAX(16) not DN_PIPE_NR_MAX(65536). config_pipe accepts 1..65536. Pipes 17-65536 created but reject EINVAL on delete — only FLUSH can remove. Permanent resource leak of kmalloc pipe+flowset+mbufs.