Add default subbuf sizes getter functions

This patch adds functions to retrieve defaults subbuf sizes. It uses the
DEFAULT_*_SUBBUF_SIZE defines from defaults.h but also make sure that
the values are at least as big as the page size.

The functions are defined as static inline in defaults.h.

Signed-off-by: Simon Marchi <simon.marchi@polymtl.ca>
Signed-off-by: David Goulet <dgoulet@efficios.com>