Fix an error in bsdinstall when ZFS installation uses a bootpool

Fix an error in bsdinstall when ZFS installation uses a bootpool

A separate bootpool is used when the selected partition type is MBR
or when the disk is GELI encrypted and a UEFI loader is selected.

BIOS avoids the bootpool by using bootcode and loader that support GELI.

bsdinstall did not remount the bootpool after creating the main pool,
which caused an error when mkdir tried to create /boot/zfs

PR:		210717
Approved by:	re (gjb)
Sponsored by:	ScaleEngine Inc.