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

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
Reviewed by: kib
MFC after: 1 week
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D31468