bgpd: move label allocation code to a specific function

bgpd: move label allocation code to a specific function

The label allocation mechanism is called implicitly for
labeled unicast paths. The check should be explicit, because
the current patch set will extend the mechanism for mpls vpn
paths, and the code should explicitly tell which safi calls
which code.

Fix this implicit call by checking the safi value. Move the
code to a specific function.

Signed-off-by: Philippe Guibert <philippe.guibert@6wind.com>