Fix grub on btrfs partition

# mount the btrfs root subvolume
mount /dev/sda1 /mnt -o subvol=/

# (optional) set the default subvolume
btrfs sub set @ /mnt

mount /proc /mnt/proc --bind
mount /dev /mnt/dev --bind
mount /sys /mnt/sys --bind

chroot /mnt

grub-install /dev/sda
update-grub /dev/sda

# activate first partition
fdisk -l /dev/sda