libbpf: fix crash in XDP socket part with new larger BPF_LOG_BUF_SIZE

In commit da11b417 ("libbpf: teach libbpf about log_level bit 2"),
the BPF_LOG_BUF_SIZE was increased to 16M. The XDP socket part of
libbpf allocated the log_buf on the stack, but for the new 16M buffer
size this is not going to work. Change the code so it uses a 16K buffer
instead.

Fixes: da11b417

 ("libbpf: teach libbpf about log_level bit 2")
Signed-off-by: Magnus Karlsson <magnus.karlsson@intel.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>