selftests/xsk: Put MAP_HUGE_2MB in correct argument

Put the flag MAP_HUGE_2MB in the correct flags argument instead of the
wrong offset argument.

Fixes: 2ddade32 ("selftests/xsk: Fix munmap for hugepage allocated umem")
Reported-by: Kal Cutter Conley <kal.conley@dectris.com>
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20230421062208.3772-1-magnus.karlsson@gmail.com