selftests/xsk: add test for too many frags

Add a test that will exercise maximum number of supported fragments.
This number depends on mode of the test - for SKB and DRV it will be 18
whereas for ZC this is defined by a value from NETDEV_A_DEV_XDP_ZC_MAX_SEGS
netlink attribute.

Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Maciej Fijalkowski <maciej.fijalkowski@intel.com> # made use of new netlink attribute
Link: https://lore.kernel.org/r/20230719132421.584801-24-maciej.fijalkowski@intel.com


Signed-off-by: Alexei Starovoitov <ast@kernel.org>