selftests: bpf: Update kfunc_param_nullable test for new error message

selftests: bpf: Update kfunc_param_nullable test for new error message

With trusted args now being the default, the NULL pointer check runs
before type-specific validation. Update test3 to expect the new error
message "Possibly NULL pointer passed to trusted arg0" instead of the
old dynptr-specific error message.

Acked-by: Eduard Zingerman <eddyz87@gmail.com>
Reviewed-by: Emil Tsalapatis <emil@etsalapatis.com>
Signed-off-by: Puranjay Mohan <puranjay@kernel.org>
Link: https://lore.kernel.org/r/20260102180038.2708325-7-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>