MFC r326278 (manu):

growfs: Commit the changes after expanding the partition

This fix the problem in arm snapshot present since at least 6 months
where growfs was failing at firstboot and dropped you in a single
user shell.

Note: In addition to this merge, kern.geom.part.mbr.enforce_chs has
been enabled on the build machine to mitigate against the issue in
the PR referenced.

PR: 226536
Sponsored by: The FreeBSD Foundation