cmd/devp2p: fix modulo in makeBlobTxs

[colinlyguo]

Finding a small inconsistency when playing with blob transactions (using this code snippet as a reference). Changing the module from 2 to 3 can make it consistent with the comments, and by the way, it can consume all blobs when calling makeBlobTxs in the tests (first tx with 2 blobs, second with 1 blob):

	var (
		t1 = s.makeBlobTxs(2, 3, 0x1)
		t2 = s.makeBlobTxs(2, 3, 0x2)
	)

Previously, the first tx only had 1 blob, and the second 1, strictly less than 3 blobs bandwidth provided.

[karalabe]

Hmmm, this code may or may not originate from a time when 0 blobs were allowed. Currently they are not.

That said, this modulo feels so weird. If the user requests N blobs, why cut it down. So strange.

CC @lightclient ?

[lightclient]

@fjl might be able to better answer, I think this is from him while he was getting the hivechain to work! My guess is the value was supposed to be 3. IUIC, the modulo allows the caller of makeBlobTxs to submit block-level values and max out at 2 blobs per tx. It's sneaky though and not noted in the function docs.

I think this could be written in a clearer way.