selftests: xsk: Fix socket creation retry

The socket creation retry unnecessarily registered the umem once for
every retry. No reason to do this. It wastes memory and it might lead
to too many pages being locked at some point and the failure of a
test.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20210922075613.12186-10-magnus.karlsson@gmail.com