Revert "Kill znode->z_gen field"
This reverts commit 4cd77889b684fd0dd1a0a995b692dda3db76a9ac. The
i_generation field in the inode is 32-bit and the SA code expects
64-bit fixed values. Revert this optimization for now until
this is cleanly addressed.
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #4538