Add large_block feature flag (WIP)

WARNING, this is a prototype patch which is only suitable for use on
dedicated test pools.  Do NOT use this patch on systems with data you
care about.  Once enabled it will cause new data to be written using
large blocks which are not understood by existing ZFS implementations.

This change is suitable for benchmarking the potential performance
impact of using large blocks.  The configurations of primarily
interest are raidz vdevs using a large number of devices.

TODO:

1) Feature flag & reference counting.  Matt has done some work that
   will allow us to quickly and easily track the number of blocks
   using a given feature.  This will be a handy for us to be able
   to be able to deactivate the feature once activated.  It would
   also be desirable to allow ZFS implementations without this
   feature to still import the pool read-only.  All datasets where
   large blocks were not enabled should still be safe to mount.

2) GRUB support.  Initially it may be best to prevent allowing
   increased block sizes on the boot filesystem (similar to how
   we handle gzip compression).  Once this functionality has been
   pushed in to grub this could be relaxed.  But it will take
   considerable time for any upstream grub improvement to make
   their way back in to the distributions.

3) For Linux these changes cannot be merged until we abandon the
   zio kmem caches.  They are unsuitable for making the required
   maximum 8M allocations.  This may have a performance impact
   on FreeBSD and Illumos as well, that needs to be investigated.

4) I've made one pass evaluating everywhere SPA_MAXBLOCKSIZE/SHIFT
   is used to determine if SPA_OLD_MAXBLKSIZE/SHIFT or
   SPA_MAXBLOCKSIZE/SHIFT should be used.  However, we'll want
   additional eyes on this and more testing to make sure it's right.
   In particular some MOS objects use blocksize=SPA_MAXBLOCKSIZE.
   We would probably want to keep those at 128k so that we have
   some hope of deactivating the feature (e.g. by destroying all
   filesystems with recordsize > 128k).

5) Investigate compatibility large block in Solaris's ZFS.  I
   strongly suspect they implemented this is a similar way.  It
   may be possible to allow read-only access those pools, or
   upgrade them to the new OpenZFS format.

6) Fully document the feature in zpool-features(5).

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>