MAP: Add optional user-supplied 'tag' field in MAPs.

MAP: Add optional user-supplied 'tag' field in MAPs.

Like other entities, allow an arbitrray user-supplied 'tag'
field to be place on created MAP domains.  It is also later
returned with the MAP details.  You might be thinking "User
assigned MAP name" here.

As the MAP domain structure was at the limit of a cacheline size,
introduce a parallel "extra data" vector with non-essential domain
information in it.

Change-Id: Icc12b64cc4cb3e040c9a475908b19f6abaf4c293
Signed-off-by: Jon Loeliger <jdl@netgate.com>