nat: Replace port refcounts with simple bitvectors

nat: Replace port refcounts with simple bitvectors

Previously, each address maintained an array of 32-bit
reference counts for each of 65K possible ports for each
of 4 NAT protocols.  Totalling 1MB per address.  Wow.

A close read of the code shows that an "is used" check
precedes each attempted reference count increment.
That means the refcount never actually gets above 1.
That in turn means algorithmically, a bit vector is
sufficient.  And one need not be allocated for more
than the highest validated port referenced.

These changes introduce a dynamically sized bit vector
replacing the reference counts, for a maximum of 32K
if all 4 protocols use port 65535.  In fact, protocol
OTHER is never used, so at most 24K will be used, and
none of it will be "statically" allocated per address.

Type: fix
Fixes: 85bee7548bc5a360851d92807dae6d4159b68314

Change-Id: I7fd70050e7bf4871692a862231f8f38cf0158132
Signed-off-by: Jon Loeliger <jdl@netgate.com>