libbpf: fix an snprintf() overflow check

libbpf: fix an snprintf() overflow check

[ Upstream commit 49e4da832d16ea4469708028731713e4b2d12ea0 ]

The snprintf() function returns the number of bytes it *would* have
copied if there were enough space.  So it can return > the
sizeof(gen->attach_target).

Fixes: da76868b0892 ("libbpf: Generate loader program out of BPF ELF file.")
Signed-off-by: Dan Carpenter <dan.carpenter@oracle.com>
Acked-by: Martin KaFai Lau <kafai@fb.com>
Link: https://lore.kernel.org/r/YtZ+oAySqIhFl6/J@kili
Signed-off-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: Sasha Levin <sashal@kernel.org>