bpf: Fix various lib and testsuite build failures on 32-bit.

Cannot cast a u64 to a pointer on 32-bit without an intervening (long)
cast otherwise GCC warns.

Signed-off-by: David S. Miller <davem@davemloft.net>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>