Add support for WRITE ATOMIC (16) command and report SBC-4 compliance.

Atomic writes are only supported for ZVOLs in "dev" mode.  In other cases
atomicity can not be guarantied and so the command is blocked.


git-svn-id: svn+ssh://svn.freebsd.org/base/head@272734 ccf9f872-aa2e-dd11-9fc8-001c23d0bc1f