finish_consuming_dead_subbuffer: fix data_size read race, reread new consumed count

Make sure finish_consuming_dead_subbuffer always see a data_size that is non
0xffffffff only when the buffer data is entirely readable, else the code rely on
the commit_seq counter which is the proper solution.

The consumed count should be re-read in each test within the loop, otherwise the
mathematic formula to get the amount of data to read does not work wrt
buffer-size wrap-around.

Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>