libbpf: fix GCC8 warning for strncpy

libbpf: fix GCC8 warning for strncpy

GCC8 started emitting warning about using strncpy with number of bytes
exactly equal destination size, which is generally unsafe, as can lead
to non-zero terminated string being copied. Use IFNAMSIZ - 1 as number
of bytes to ensure name is always zero-terminated.

Signed-off-by: Andrii Nakryiko <[email protected]>
Cc: Magnus Karlsson <[email protected]>
Acked-by: Yonghong Song <[email protected]>
Acked-by: Magnus Karlsson <[email protected]>
Signed-off-by: Daniel Borkmann <[email protected]>