tools: bpftool: remember to close the libbpf object after prog load

tools: bpftool: remember to close the libbpf object after prog load

Remembering to close all descriptors and free memory may not seem
important in a user space tool like bpftool, but if we were to run
in batch mode the consumed resources start to add up quickly.  Make
sure program load closes the libbpf object (which unloads and frees
it).

Fixes: 7424c14fffe7 ("bpftool: implement prog load command")
Signed-off-by: Jakub Kicinski <jakub.kicinski@netronome.com>
Reviewed-by: Quentin Monnet <quentin.monnet@netronome.com>
Acked-by: Song Liu <songliubraving@fb.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>