Expand a btrfs partion size on a loopback device

Expand the source loopback file (Change of and seek in the right hand side)

dd if=/dev/zero of=file_name.img bs=1 count=0 seek=100G

Update the size of the loopback device (Change the loopback device name at the tail as appropriate)

losetup -c /dev/loop3

Expand the btrfs (Change the mount point at the tail as appropriate)

btrfs filesystem resize max /mnt/mount_point