id-pool: Refactor id_pool_alloc_id to allow any 32 bit value to be an id

id_pool_alloc_id() was created by breaking out the recirculation
allocation code. As it is now a library call it makes sense to remove
the restriction that id 0 is reserved.

Signed-off-by: Simon Horman <simon.horman@netronome.com>
Signed-off-by: Ben Pfaff <blp@nicira.com>