btrfs: avoid double slot decrement at btrfs_convert_free_space_to_extents()

There's no need to subtract 1 from path->slots[0] and then decrement the
slot, we can reduce the generated assembly code by decrementing the slot
and then use it.

Module size before:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1846220	 162998	  16136	2025354	 1ee78a	fs/btrfs/btrfs.ko

Module size after:

  $ size fs/btrfs/btrfs.ko
     text	   data	    bss	    dec	    hex	filename
  1846204	 162998	  16136	2025338	 1ee77a	fs/btrfs/btrfs.ko

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>

Author: fdmanana

fs/btrfs/free-space-tree.c

@@ -406,12 +406,12 @@ int btrfs_convert_free_space_to_extents(struct btrfs_trans_handle *trans,
 				data_size = free_space_bitmap_size(fs_info,
 								found_key.offset);
 
-				ptr = btrfs_item_ptr_offset(leaf, path->slots[0] - 1);
+				path->slots[0]--;
+				ptr = btrfs_item_ptr_offset(leaf, path->slots[0]);
 				read_extent_buffer(leaf, bitmap_cursor, ptr,
 						   data_size);
 
 				nr++;
-				path->slots[0]--;
 			} else {
 				ASSERT(0);
 			}