svm: store normalized head/tail for fifo

svm: store normalized head/tail for fifo

If head/tail are stored as "absolute" values that are normalized to [0,
fifo_size] interval, when fifo is shrunk/grown the consumer and producer
have to independently update to the new fifo size and fix head and tail,
respectively.

If the head and tail are stored as normalized values, under the right
conditions, they don't need to be fixed when fifo size changes.

This reverts one of the changes in gerrit 18223.

Change-Id: I55a908828afe90925cf7c20186a940b25e5805f9
Signed-off-by: Florin Coras <fcoras@cisco.com>