bpf: remove reference to sock_filter_ext from kerneldoc comment

struct sock_filter_ext didn't make it into the tree and is now called
struct bpf_insn. Reword the kerneldoc comment for bpf_convert_filter()
accordingly.

Signed-off-by: Tobias Klauser <tklauser@distanz.ch>
Acked-by: Daniel Borkmann <daniel@iogearbox.net>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Signed-off-by: David S. Miller <davem@davemloft.net>