Add support for MapExtra and bloom filter maps

[ti-mo]

Kernel 5.16 saw the introduction of the bloom filter map type: https://lore.kernel.org/all/20211027234504.30744-1-joannekoong@fb.com/.

This also included new map_extra field for maps that need more configuration than the existing map properties.

In #668, selftests relying on MapExtra are ignored for now.

Maybe we should not expose the field from MapSpec immediately, until we figure out how to expose it as a user-friendly API.