Fix inode birthtime updating logic.

The birthtime field of struct vattr does not checked
for VNOVAL in case of ext2_setattr() and produce incorrect
inode birthtime values.

Found using pjdfstest:

pjdfstest/tests/utimensat/03.t

Reviewed by: pfg
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D29929