Make linux(4) use kern_socketpair(9)

[trasz]

Summary

Make linux(4) use kern_socketpair(9) instead of going through
sys_socketpair(). It's a cleanup; no functional changes.

[kib]

sys/compat/linux/linux_socket.c

758

Wrong indent.

770

else is not needed. Is this correct, in fact ?

776

Extra empty line.

782

Same.

[trasz]

Fix things pointed out by kib@.

[kib]

sys/compat/linux/linux_socket.c

766

Does this sentence in comment still makes sense ?

While there, remove the blank line before the comment.

775

sizeof(sv) is better IMO.