netdump: Fix 11 compatibility DIOCSKERNELDUMP ioctl

[cem]

Summary

The logic was present for the 11 version of the DIOCSKERNELDUMP ioctl, but
had not been updated for the 12 ABI.

[iz-rpi03_hs-karlsruhe.de]

Now a kernel configuration without COMPAT_FREEBSD11 generates the following errror:

`--- netdump_client.o ---
/usr/src/sys/netinet/netdump/netdump_client.c:1143:8: error: unused variable 'u' [-Werror,-Wunused-variable]

u_int u;
      ^

1 error generated.
`

head/sys/netinet/netdump/netdump_client.c

1143

Use #ifdef COMPAT_FREEBSD11 here as well.

[cem]

head/sys/netinet/netdump/netdump_client.c

1143

Thanks for the catch!

[iz-rpi03_hs-karlsruhe.de]

LGTM.
A kernel configuration without COMPAT_FREEBSD11 is building successfully again.