xdp: Handle MEM_TYPE_XSK_BUFF_POOL correctly in xdp_return_buff()

It turns out that it does exist a path where xdp_return_buff() is
being passed an XDP buffer of type MEM_TYPE_XSK_BUFF_POOL. This path
is when AF_XDP zero-copy mode is enabled, and a buffer is redirected
to a DEVMAP with an attached XDP program that drops the buffer.

This change simply puts the handling of MEM_TYPE_XSK_BUFF_POOL back
into xdp_return_buff().

Fixes: 82c41671 ("xdp: Simplify xdp_return_{frame, frame_rx_napi, buff}")
Reported-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Signed-off-by: Björn Töpel <bjorn.topel@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Maxim Mikityanskiy <maximmi@nvidia.com>
Link: https://lore.kernel.org/bpf/20201127171726.123627-1-bjorn.topel@gmail.com