btrfs: assert we can NOCOW the range in btrfs_truncate_block()

We call btrfs_check_nocow_lock() to see if we can NOCOW a block sized
range but we don't check later if we can NOCOW the whole range.
It's unexpected to be able to NOCOW a range smaller than blocksize, so
add an assertion to check the NOCOW range matches the blocksize.

Reviewed-by: Boris Burkov <boris@bur.io>
Signed-off-by: Filipe Manana <fdmanana@suse.com>
Signed-off-by: David Sterba <dsterba@suse.com>