The variable 'last_index' is calculated in the __btrfs_buffered_write
function and passed as a parameter to the prepare_pages function,
but is not used anywhere in the prepare_pages function.

Remove instances of 'last_index' in these functions.
Signed-off-by: NMitch Harder <mitch.harder@sabayonlinux.org>
Signed-off-by: NChris Mason <chris.mason@oracle.com>