From: Brian Behlendorf <behlendorf1@llnl.gov>
Subject: [PATCH] feature commit cb

DMU commit callbacks. Provides an API to attach callbacks to transactions,
which are called by the DMU when they are safely committed to disk.
See Lustre bug 14117 for details.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>