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
Sponsored by: The FreeBSD Foundation

(cherry picked from commit eca9ac5a32e432313b1c7f52f43dd11504fceef4)