f2fs: optimise the truncate_data_blocks_range() range

f2fs: optimise the truncate_data_blocks_range() range

The function truncate_data_blocks_range() decrements the valid
block count of inode via dec_valid_block_count(). Since this
function updates the i_blocks field of inode, we can update this
field once we have calculated total the number of blocks
to be freed.

Therefore we can decrement valid blocks outside of the for loop.

	if (nr_free) {
+		dec_valid_block_count(sbi, dn->inode, nr_free);
 		set_page_dirty(dn->node_page);
 		sync_inode_page(dn);
 	}

'nr_free' tells the total number of blocks freed. So, we can
just directly pass this value to dec_valid_block_count() and update
the i_blocks.

Signed-off-by: Namjae Jeon <namjae.jeon@samsung.com>
Signed-off-by: Pankaj Kumar <pankaj.km@samsung.com>
Signed-off-by: Jaegeuk Kim <jaegeuk.kim@samsung.com>