User session counters stay <= per-user limit

User session counters stay <= per-user limit

When a user session is allocated/reused, only increase
one of the session counters for that user if the counters
are below the per-user limit.

THis addresses a SEGV that arises after the following
sequence of events:

- an outside interface IP address is put in a pool
- a user exceeds the number of per-user translations by
an amount greater than the number of per-user translations
(nsessions + nstaticsessions > 100 + 100)
- the outside interface IP address is deleted and then added
again (observed when using DHCP client, likely happens if
address changed via CLI, API also)
- the user sends more packets that should be translated

When nsessions is > the per-user limit,
nat_session_alloc_or_recycle() reclaims the oldest existing
user session. When an outside address is deleted, the
corresponding user sessions are deleted. If the counters were
far above the per-user limit, the deletions wouldn't result
in the counters dropping back below the limit. So no session
could be reclaimed -> SEGV.

Change-Id: I940bafba0fd5385a563e2ce87534688eb9469f12
Signed-off-by: Matthew Smith <mgsmith@netgate.com>