bpf: Avoid warning when re-casting __bpf_call_base into __bpf_call_base_args

BPF interpreter uses extra input argument, so re-casts __bpf_call_base into
__bpf_call_base_args. Avoid compiler warning about incompatible function
prototypes by casting to void * first.

Fixes: 1ea47e01

 ("bpf: add support for bpf_call to interpreter")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Yonghong Song <yhs@fb.com>
Link: https://lore.kernel.org/bpf/20210112075520.4103414-3-andrii@kernel.org