libbpf: fix BPF_LOG_BUF_SIZE off-by-one error

The BPF_PROG_LOAD condition for kernel version <= 5.1 is

   log->len_total > UINT_MAX >> 8 /* (16 * 1024 * 1024) - 1 */

Signed-off-by: McCabe, Robert J <robert.mccabe@rockwellcollins.com>
Signed-off-by: Alexei Starovoitov <ast@kernel.org>