Minor fixes
Updated ioctls though it seems they are unused.
Tested with zvols of varying volblocksize, with compression=off, zle, and lz4, and formatted as a JHFS+ volume.
Regardless of compression setting, used space accurately drops when files are deleted.
At times, there is a delay between filesystem syncs, up to 30 seconds or less. When the next doSync is called the blocks are unmapped.

Compared results with master - zvols retain all used blocks unless ‘Erase free space’ or ’Secure erase’ are used. This works fine with small zvols, but can be tedious on large ones.