Revert "xdp: add NULL pointer check in __xdp_return()"

This reverts commit 36e0f12b.

The reverted commit adds a WARN to check against NULL entries in the
mem_id_ht rhashtable. Any kernel path implementing the XDP (generic or
driver) fast path is required to make a paired
xdp_rxq_info_reg/xdp_rxq_info_unreg call for proper function. In
addition, a driver using a different allocation scheme than the
default MEM_TYPE_PAGE_SHARED is required to additionally call
xdp_rxq_info_reg_mem_model.

For MEM_TYPE_ZERO_COPY, an xdp_rxq_info_reg_mem_model call ensures
that the mem_id_ht rhashtable has a properly inserted allocator id. If
not, this would be a driver bug. A NULL pointer kernel OOPS is
preferred to the WARN.

Suggested-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Acked-by: Jesper Dangaard Brouer <brouer@redhat.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>