btrfs: avoid double slot decrement at btrfs_convert_free_space_to_extents()

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>