selftest/bpf: Check invalid length in test_xdp_update_frags

Update test_xdp_update_frags adding a test for a buffer size
set to (MAX_SKB_FRAGS + 2) * PAGE_SIZE. The kernel is supposed
to return -ENOMEM.

Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/3e4afa0ee4976854b2f0296998fe6754a80b62e5.1644366736.git.lorenzo@kernel.org