Fix: get_subbuf: bound number of consumerd retry

When an application traced with per-UID buffers vanishes between its
reserve and commit ring buffer operations (e.g. segfault), it may cause
the consumer daemon to enter an endless loop, awaiting for the
application to complete its commit.

Fix this situation by bounding the number of retry using an adaptative
approach: try actively for a few times, and then try a few more times
with a 10ms delay between attempts.

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