paging: Use bsub->active_paging_requests to allow early loop termination adding paging_req

Before this patch, the entire queue of paging_request had to be iterated
in order to find if the subscriber already had an active paging request
(discarding duplicates).

Now that bsc_subscriber holds a list of its active paging requests, it's
easier to look at its list to find out if it is already active on that
BTS.

As a result, there's no need to unconditionally iterate the whole list
and we can optimize the loop finding the spot to insert the new queue:
- First the potentially way smaller loop over
  bsub->active_paging_requests is done
- Second, only if the first loop allowed, the bts->pending_requests is
  iterated and potentially early terminated.

Related: SYS#6200
Change-Id: I7912275026c4d4983269c8870aa5565c93277c5a
---
M src/osmo-bsc/paging.c
1 file changed, 28 insertions(+), 17 deletions(-)