svm: basic support for fifo shrinking

svm: basic support for fifo shrinking

As opposed to growing, this is not a bulk operation, instead dependent
on how the producer/consumer advance head and tail, the fifo will shrink
in one or multiple steps.

Only once the fifo's nitems and size are reduced to their appropriate
values, equal or larger to what was requested, can the fifo chunks be
collected by the owner. Chunk collection must be done with the segment
heap pushed.

Change-Id: Iae407ccf48d85320aa3c1e0304df56c5972c88c1
Signed-off-by: Florin Coras <fcoras@cisco.com>