bpf samples: fix compiler errors with sockex2 and sockex3

These samples fail to compile as 'struct flow_keys' conflicts with
definition in net/flow_dissector.h. Fix the same by renaming the
structure used in the sample.

Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>