consumerd: move address computation from on_read_subbuffer_mmap

The computation of the subbuffer's address is moved outside of
lttng_consumer_on_read_subbuffer_mmap to make it usable with a regular
buffer. This facilitates an upcoming change.

Moreover this has the benefit of isolating domain-specific logic from
this function which is supposed to be domain-agnostic.

Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>
Change-Id: I16f8ccaa73804f98fa03e69136548e6d6b7782e5