initramfs: use correct dataset for rootfs on rollback=1

When booting with root=zfs:rpool/myrootfs@foosnapshot rollback=1,
myrootfs and its descendants get rolled back to foosnapshot, however
ZFS_BOOTFS still contains myrootfs@foosnapshot instead of the
actually desired value of myrootfs.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: John Kennedy <john.kennedy@delphix.com>
Signed-off-by: Daniel Reichelt <hacking@nachtgeist.net>
Closes #12585
Closes #12586