bpf: Fix bpf_dynptr_slice{_rdwr} to return NULL instead of 0

Change bpf_dynptr_slice and bpf_dynptr_slice_rdwr to return NULL instead
of 0, in accordance with the codebase guidelines.

Fixes: 66e3a13e ("bpf: Add bpf_dynptr_slice and bpf_dynptr_slice_rdwr")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Joanne Koong <joannelkoong@gmail.com>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20230302053014.1726219-1-joannelkoong@gmail.com