Btrfs: Remove unused variable 'last_index' in file.c

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: Mitch Harder <[e-mail]>
Signed-off-by: Chris Mason <[e-mail]>

fs/btrfs/file.c

···
 static noinline int prepare_pages(struct btrfs_root *root, struct file *file,
 			 struct page **pages, size_t num_pages,
 			 loff_t pos, unsigned long first_index,
-			 unsigned long last_index, size_t write_bytes)
+			 size_t write_bytes)
 {
 	struct extent_state *cached_state = NULL;
 	int i;
···
 	struct btrfs_root *root = BTRFS_I(inode)->root;
 	struct page **pages = NULL;
 	unsigned long first_index;
-	unsigned long last_index;
 	size_t num_written = 0;
 	int nrptrs;
 	int ret = 0;
···
 		return -ENOMEM;
 
 	first_index = pos >> PAGE_CACHE_SHIFT;
-	last_index = (pos + iov_iter_count(i)) >> PAGE_CACHE_SHIFT;
 
 	while (iov_iter_count(i) > 0) {
 		size_t offset = pos & (PAGE_CACHE_SIZE - 1);
···
 		 * contents of pages from loop to loop
 		 */
 		ret = prepare_pages(root, file, pages, num_pages,
-				    pos, first_index, last_index,
-				    write_bytes);
+				    pos, first_index, write_bytes);
 		if (ret) {
 			btrfs_delalloc_release_space(inode,
 					num_pages << PAGE_CACHE_SHIFT);