bpf: Fix a erroneous check after snprintf()

bpf: Fix a erroneous check after snprintf()

snprintf() does not return negative error code on error, it returns the
number of characters which *would* be generated for the given input.

Fix the error handling check.

Fixes: 57539b1c0ac2 ("bpf: Enable annotating trusted nested pointers")
Signed-off-by: Christophe JAILLET <christophe.jaillet@wanadoo.fr>
Link: https://lore.kernel.org/r/393bdebc87b22563c08ace094defa7160eb7a6c0.1694190795.git.christophe.jaillet@wanadoo.fr
Signed-off-by: Alexei Starovoitov <ast@kernel.org>