selftests: bpf: Update failure message for rbtree_fail

selftests: bpf: Update failure message for rbtree_fail

The rbtree_api_use_unchecked_remove_retval() selftest passes a pointer
received from bpf_rbtree_remove() to bpf_rbtree_add() without checking
for NULL, this was earlier caught by __check_ptr_off_reg() in the
verifier. Now the verifier assumes every kfunc only takes trusted pointer
arguments, so it catches this NULL pointer earlier in the path and
provides a more accurate failure 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/20260102180038.2708325-8-puranjay@kernel.org
Signed-off-by: Alexei Starovoitov <ast@kernel.org>