recreated zvol-unmap topic, branched from master
cstyle fixes
only exception: 32 lines longer than 80 chars in zvol.c
minor fix
restore const to correct location
Force termination of zvol devices
Remove unused performRead and performWrite
cstyle fixes
./scripts/cstyle.sh -cp include/sys/zvol* module/zfs/zvol* shows zero issues

Additional style cleanups
visually located issues, cstyle.sh still shows none.
Merge pull request #199 from openzfsonosx/zvol-unmap

ZVOL unmap