Changes

stream: Add osmo_stream_srv_link_set_msgb_alloc_info()

This is needed so that user can set desired headroom+size info on
rx-allocated msgbs, so that osmo_stream_srv can inherit them upon
osmo_stream_srv_create2().

Manually obtaining the iofd through osmo_stream_srv_get_iofd() and
changing the params directly in the iofd produces undesired effects,
since the first msgbs are alredy allocated during
osmo_stream_srv_create2 (which calls osmo_iofd_register()) before the
object is available to the user.
As a result, first rx messages may come with unexpected size/headroom.

Ideally we'd had written the osmo_stream_srv APIs to have a 2 step
setup, eg _alloc() + _run()/_start(), but too late to do so now.
This approach is already follwed by existing
osmo_stream_srv_link_set_tx_queue_max_length() API.

Change-Id: I80a1c4b227629e3ca0c8c587a103db6057322cb4