Fix: Consumer return value check after mmap/splice

With the feature of not sending the trace file padding over the network
for streaming, the return value of mmap or splice failed to match the
passed value when the trace is written on disk since we ignore the
padding for that case.

However, this was simply to print an error message so no behavior is
changed with this patch.

Detail comments were added to explain this "complex" return value check.

Signed-off-by: David Goulet <dgoulet@efficios.com>