ktrace: fix a race between writes and close

[kib]

Summary

It was possible that termination of ktrace session occured during some record write, in which case write occured after the close of the vnode. Use ktr_io_params refcounting to avoid this situation, by taking the reference on the structure instead of vnode.

[markj]

sys/kern/kern_ktrace.c

1360

So we disable tracing if the file is closed and re-opened, i.e., ABA-like race. I think that is ok.

[kib]

sys/kern/kern_ktrace.c

1360

Yes it is same as before ktr_io_params introduction, and I decided to keep it that way from that time.