Should the argument be unsigned?
Then perhaps you could use fls(val) - 1 (please double-check me).
Also might be a good idea to put it into e.g. sys/libkern.h for other users, I did not found existing analog in our sources.

Why not use powerof2(x) from sys/param.h ?

I could use fls() as you suggest which would lead to

#define ILOG2(num) ((1 << (fls(num) - 1) == num) ? (fls(num) - 1) : -1)

or I could create a new function which would not end up calling fts twice. It needs to be used in both kernel and user compiles, so it would have to be in both libkern.h and also added to ffs.3. Not sure that it is useful enough to do that. What do you think?

I did not know about powerof2() in sys/param.h. I only looked for the all-cap version since that is what the style guide says such non-function things should be :-) Now using powerof2()

This is a strange function, I must admit. So do you want to get a logarithm if the number is just power of 2, and -1 otherwise? Or do you want the log2 always, except for zero?

I thought that you wanted

#define ILOG2(num) (fls(num) - 1)

and that was my proposal. Your version can be written to use fls only once using GNU extension:

#define ILOG2(num) __extention__ ({
   unsigned _f = fls(num) - 1;
   (1U << _f) == num ? _f : -1;
})

In this variant, it should be 1U << (...) I believe, otherwise you get undefined behavior.

I realize that just using

#define ILOG2(num) (fls(num) - 1)

is actually fine since I have already checked that the num argument is a power of 2 all I need to verify is that ILOG2 is correct. I have made that update in my latest diff.

Are there enough checks to ensure that INOPF() != 0?

We check that fs_bsize >= MINBSIZE (4096).
UFS2 has the largest inodes sizeof(struct ufs2_dinode) (256).
We check that fs_inopb == (fs_bsize / sizeof(struct ufs2_dinode)) which must be at least 16 (4096 / 256).
We check that fs_frag <= MAXFRAG (8) and fs_fragshift is ILOG2(fs_frag).
Since INOPF is fs_inopb >> fs_fragshift it must be at least 2.
Another way to look at it is that the minimum fs_fsize is 512 which would hold 2 UFS2 inodes.