Fix register consumer fd test on registration

When a kernel session is created, a consumer is immediately spawned. So,
using two sessions, the second one was not able to register it's own
consumer since the consumer_fd is set because there is a running kernel
consumer from the first created session. The flag consumer_fds_sent is
now used to test if the kernel consumer is already consuming buffers.

Reported-by: Julien Desfossez <julien.desfossez@polymtl.ca>
Signed-off-by: David Goulet <david.goulet@polymtl.ca>