Fix: initialize kernel stream max subbuffer size on creation

A consumer stream's maximal subbuffer size is only initialized while
taking a snapshot. This unexpectedly leaves the max_sb_size at 0
causing the rotation target sequence number computation to crash with
a SIGFPE.

This fix initializes the value as soon as possible to provide a
behaviour in line with that of the ust-consumer.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>