MINOR: ncbuf: optimize storage for the last gap

MINOR: ncbuf: optimize storage for the last gap

Relax the constraint for gap storage when this is the last block.
ncb_blk API functions will consider that if a gap is stored near the end
of the buffer, without the space to store its header, the gap will cover
entirely the buffer end.

For these special cases, the gap size/data size are not write/read
inside the gap to prevent an overflow. Such a gap is designed in
functions as "reduced gap" and will be flagged with the value
NCB_BK_F_FIN.

This should reduce the rejection on future add operation when receiving
data in-order. Without reduced gap handling, an insertion would be
rejected if it covers only partially the last buffer bytes, which can be
a very common case.