Btrfs-progs: check all slots in leaves

There's an off by one error in btrfs_check_leaf, we should be going to nritems -
1, not nritems - 2, we were missing problems with items in the very last slot.
Thanks,

Signed-off-by: Josef Bacik <jbacik@fb.com>
Signed-off-by: David Sterba <dsterba@suse.cz>