btrfs: simplify range end calculations in truncate_block_zero_beyond_eof()

The way zero_end is calculated and used does a -1 and +1 that
effectively cancel out, so this can be simplified. This is also
preparatory patch for using a helper for folio_pos + folio_size with the
semantics of exclusive end of the range.

Reviewed-by: Johannes Thumshirn <johannes.thumshirn@wdc.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: kdave

fs/btrfs/inode.c

@@ -4819,9 +4819,9 @@ static int truncate_block_zero_beyond_eof(struct btrfs_inode *inode, u64 start)
 	 */
 
 	zero_start = max_t(u64, folio_pos(folio), start);
-	zero_end = folio_pos(folio) + folio_size(folio) - 1;
+	zero_end = folio_pos(folio) + folio_size(folio);
 	folio_zero_range(folio, zero_start - folio_pos(folio),
-			 zero_end - zero_start + 1);
+			 zero_end - zero_start);
 
 out_unlock:
 	folio_unlock(folio);