samples, bpf: Add missing munmap in xdpsock

We mmap the umem region, but we never munmap it.
Add the missing call at the end of the cleanup.

Fixes: 3945b37a

 ("samples/bpf: use hugepages in xdpsock app")
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Björn Töpel <bjorn.topel@intel.com>
Link: https://lore.kernel.org/bpf/20210303185636.18070-3-maciej.fijalkowski@intel.com