hammer2 - pfsmount -> clustermount separation part 2
* Further separate the high-level VNOPS/inode (hammer2_pfsmount) layer
from the lower level device (hammer2_mount, hammer2_chain) layer.
* Remove hmp fields from hammer2_trans and hammer2_inode.
* Add hammer2_cluster to the pfsmount as degenerate case for now. This
will be used to list all devices backing the PFS mount, pertaining to
the copies mechanism.
* Run all logical (file) buffer cache operations through the device buffer
cache. Remove previous direct-mapped shortcuts and disable BMAP for now.
Basically the issue here is that with multiple devices backing a HAMMER2
mount, the normal file buffer cache 'cached disk offset' operations used
to shortcut I/O just won't work. We can add the shortcut back in later
for single-backing-device mounts but for now separate them out entirely
and bcopy() between them.
* This will also make it easier for the GSOC H2 file compression project.
* Restore some of the lost performance by using the newly implemented
cluster_readcb() buffer cache function.