btrfs: use the right number of levels for lockdep keysets
BTRFS_MAX_LEVEL is 8 and the keyset table is supposed to have a key for
each level, but we'll never have more than 8 levels. The values passed
to btrfs_set_buffer_lockdep_class are always derived from a valid extent
buffer. Set the array sizes to the right value.
Reviewed-by: Josef Bacik <josef@toxicpanda.com> Signed-off-by: David Sterba <dsterba@suse.com>