Add default subbuf sizes getter functions

Add default subbuf sizes getter functions

commit 8b3bd7a32e514aae56e718491a10f582b03140c3 upstream.

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.

[ Edit by Christian Babeux: Resolved conflict in src/common/Makefile.am
  and src/common/defaults.h ]

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