Btrfs-progs: remove incorrect slot decrement

Btrfs-progs: remove incorrect slot decrement

In btrfs_set_block_flags() we want to check if the slot
in the leaf points to the first item in the leaf - if it
doesn't check if the previous item in the leaf is an extent
item. By removing this extra slot decrement we are indeed
checking the item right before the slot, and not the second
item before.

Signed-off-by: Filipe David Borba Manana <fdmanana@gmail.com>
Reviewed-by: Josef Bacik <jbacik@fusionio.com>
Signed-off-by: David Sterba <dsterba@suse.cz>
Signed-off-by: Chris Mason <chris.mason@fusionio.com>