nanobsd: Prefer nano_umount to umount

nano_umount[^1] is a convenience routine used to override issues with
umount(8) without changing the nanobsd.sh source code. There were a few
places where it was not being used.

[^1]: Introduced in 6d12b61a88a6 ("Add a routine for easy workaround any
umount issues w/o hacking nanobsd.sh.")

Reviewed by: imp, emaste
MFC after: 1 week
Differential Revision: https://reviews.freebsd.org/D48780