return (VOP_BMAP(...

Do you need such trivial helper at all ? IMO you can directly call VOP_BMAP() in vn_ioctl().

We typically indent as if MAC == 1 always (look at other similar places).

I did it to avoid bloating vn_ioctl, and to separate the concerns of parsing com from implementing an ioctl. Would you be happier with the original approach if I used some other naming convention for vn_ioc_bmap2? I see that there are other functions in this file that take a struct file* argument too, like vn_read and vn_write.

FIONREAD case looks fine and not bloated. In the vn_ioc_bmap2() interface, I do not like that the file * is passed where function really operates on vnode, and that the ABI structure is passed as an argument to kernel helper. This does not depend on name.

FWIW, vn_read, or better, vn_write, operate on a lot of state in file instead of only vnode.