tools, bpftool: Add missing close before bpftool net attach exit

progfd is created by prog_parse_fd() in do_attach() and before the latter
returns in case of success, the file descriptor should be closed.

Fixes: 04949ccc ("tools: bpftool: add net attach command to attach XDP on interface")
Signed-off-by: Wang Hai <wanghai38@huawei.com>
Signed-off-by: Daniel Borkmann <daniel@iogearbox.net>
Link: https://lore.kernel.org/bpf/20201113115152.53178-1-wanghai38@huawei.com