VOP_STAT: Provide a default value for va_gen

[markj]

Summary

Some filesystems, e.g., pseudofs and the NFSv3 client, do not provide
one.

Reported by: KMSAN

[kib]

sys/kern/vfs_default.c

1503

I believe that it is better to initialize the field with VNOVAL AKA -1.

For instance, UFS *set*attr compares va_gen with VNOVAL.

[markj]

sys/kern/vfs_default.c

1503

I note that quite a few filesystems individually initialize va_gen as 0 in their getattr implementations. Should they be left alone?

[kib]

sys/kern/vfs_default.c

1503

Hm, UFS interprets 0 as not having i_gen initialized, not VNOVAL. See ffs_vfsops.c:ffs_vgetf().

So now I think that my suggestion is wrong, and VNOVAL is the valid value for va_gen e.g. from UFS.