We encountered a weird random kernel initrd unpacking error on btrfs
and finally found it was caused by incorrect address reference in range
check for type GRUB_BTRFS_EXTENT_REGULAR and the entire result is
unpredictable.
This is a quick fix to make the address reference to the
grub_btrfs_extent_data structure correctly, not the pointer variable
to it.
Any suggestions to this patch is welcome.
