bpf: remove redundant variable old_flags

Variable old_flags is being assigned but is never read; it is redundant
and can be removed.

Cleans up clang warning: Value stored to 'old_flags' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Signed-off-by: David S. Miller <davem@davemloft.net>