vfs: Avoid a comparison with an uninitialized field in setutimes()

[markj]

Summary

Some filesystems, e.g., devfs, do not populate va_birthtime in their
GETATTR implementations. To handle this, make sure that va_birthtime is
initialized to the quasi-standard value of { VNOVAL, 0 } before calling
VOP_GETATTR.

Reported by: KMSAN
Sponsored by: The FreeBSD Foundation

[kib]

I am fine with this, but also wonder if it makes sense to fill birthtime for devfs. We can record the make_dev() time in cdev_priv inside devfs_alloc()

[markj]

To be clear, this change is mimicking vop_stdstat(), which pre-initializes several fields explicitly.

In D31468#709284, @kib wrote:

I am fine with this, but also wonder if it makes sense to fill birthtime for devfs. We can record the make_dev() time in cdev_priv inside devfs_alloc()

I can't see any problem with that, but I am not sure whether it would be useful to anyone vs. simply initializing va_birthtime to {-1, 0} in devfs_getattr().