test_bpf: test LD_ABS and LD_IND instructions on fragmented skbs.

These new tests exercise various load sizes and offsets crossing the
head/fragment boundary.

Signed-off-by: Nicolas Schichan <nschichan@freebox.fr>
Acked-by: Alexei Starovoitov <ast@plumgrid.com>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>