selftests: xsk: Disassociate umem size with packets sent

Disassociate the number of packets sent with the number of buffers in
the umem. This so we can loop over the umem to test more things. Set
the size of the umem to be a multiple of 2M. A requirement for huge
pages that are needed in unaligned mode.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Link: https://lore.kernel.org/bpf/20210825093722.10219-8-magnus.karlsson@gmail.com