First and foremost, fellow_log_prep_max_regions was defined wrong:

Except in fellow_cache_test, we call log submission with a maximum of
FELLOW_DISK_LOG_BLOCK_ENTRIES = 56 DLEs. The intention of the
fellow_log_prep_max_regions was was to allocate space to track return
of the maximum number of regions possibly contained. The exact maximum
would be (FELLOW_DISK_LOG_BLOCK_ENTRIES - 1) * DLE_REG_NREGION + 1 =
(55 * 4) + 1 = 221, which is higher than FELLOW_DISK_LOG_BLOCK_ENTRIES
* DLE_BAN_REG_NREGION = 56 * 3 = 168.

Yet it seems prudent to not reply on any fixed maximum, and also our
test cases call for a higher value, so we now define the maximum three
times the actually used value, and also ensure that we batch the code
to this size.

In addition, one assertion in fellow_log_entries_prep() was wrong (it
compared a number of DLEs with a number of regions).

We also tighten some assertions to help future analysis of possible
issues in this area:

- Ensure that the data path via fellow_log_entries_prep() only ever
  uses a region list on the stack.

- By using the regionlist_onlystk_add() macro, ensure that we hit an
  assertion on the array on stack, rather than one on the regionlist
  pointer.

Diff best viewed with -b

Fixes #18