Fixed bsdinstall location of vfs.zfs.min_auto_ashift

vfs.zfs.min_auto_ashift is a sysctl only not a tunable so updated bsdinstall
to use the correct location /etc/sysctl.conf instead of /boot/loader.conf

Reported by: Aaron Caza
Reviewed by: allanjude
MFC after: 2 days
Sponsored by: Multiplay
Differential Revision: https://reviews.freebsd.org/D11278