btrfs: convert BUG_ON() in btrfs_truncate_inode_items to ASSERT

We have a correctness BUG_ON() in btrfs_truncate_inode_items to make
sure that we're always using min_type == BTRFS_EXTENT_DATA_KEY if
new_size is > 0.  Convert this to an ASSERT.

Reviewed-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: Josef Bacik <josef@toxicpanda.com>
Reviewed-by: David Sterba <dsterba@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>