hostpath: Add block volume support

[darkowlzz]

This change adds block volume support to hostpath driver.

When a block volume request is received, a block file is created at
provisionRoot with the requested capacity as size.

At node publish, a loop device is created associated with the block
file. This loop device is then symlinked to the publish target path.
For read only volume publish, the loop device is created with read only
flag.

At node unpublish, the symlinked file is deleted, and the loop device is
detached from the block file.

At volume delete, the block file is deleted.

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[pohly]

Wow, that was fast. Thanks a lot @darkowlzz.

Before this can be merged we need to get the hostpath driver moved (kubernetes-csi/csi-driver-host-path#1) and then also move this PR. In the meantime we can discuss it here.

@lpabon: I remember that you were raising the question of handling block devices with symlinks vs. bind mounts. What was the outcome? Is the approach taken here correct?

[pohly]

/hold

I'm holding this because we don't want to add any further changes here. It's still ready for review.

[pohly]

@darkowlzz Please move this PR over to the csi-driver-host-path repo. The code was merged into master of that repo.

[darkowlzz]

Moved to kubernetes-csi/csi-driver-host-path#6