Convert the last remaining 256 byte "small"bufs to 128 bytes.

Convert the last remaining 256 byte "small"bufs to 128 bytes.
(The actual size doesn't matter, as the buffers are only there to
save a malloc() for the common, short, case. Coverage reports suggest
that we aren't actually testing the long case. Yet - will fix this)

p4raw-id: //depot/perl@29987