bpf: Suppress 'passing zero to PTR_ERR' warning

Kernel Test Robot complains about passing zero to PTR_ERR for the said
line, suppress it by using PTR_ERR_OR_ZERO.

Fixes: c0a5a21c ("bpf: Allow storing referenced kptr in map")
Reported-by: kernel test robot <lkp@intel.com>
Signed-off-by: Kumar Kartikeya Dwivedi <memxor@gmail.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20220521132620.1976921-1-memxor@gmail.com