selftests/xsk: Remove unnecessary parameter from pkt_set() function call

The pkt_set() function no longer needs the umem parameter. This commit
removes the umem parameter from the pkt_set() function.

Signed-off-by: Tushar Vyavahare <tushar.vyavahare@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Magnus Karlsson <magnus.karlsson@intel.com>
Link: https://lore.kernel.org/bpf/20230927135241.2287547-6-tushar.vyavahare@intel.com