Improve TXG handling in the TRIM module.

This patch adds some improvements to the way the trim module considers
TXGs:

 - Free ZIOs are registered with the TXG from the ZIO itself, not the
   current SPA syncing TXG (which may be out of date);
 - L2ARC are registered with a zero TXG number, as L2ARC has no concept
   of TXGs;
 - The TXG limit for issuing TRIMs is now computed from the last synced
   TXG, not the currently syncing TXG. Indeed, under extremely unlikely
   race conditions, there is a risk we could trim blocks which have been
   freed in a TXG that has not finished syncing, resulting in potential
   data corruption in case of a crash.

Reviewed by:	pjd (mentor)
Approved by:	pjd (mentor)
Obtained from:	https://github.com/dechamps/zfs/commit/5b46ad40d9081d75505d6f3bf04ac652445df366
MFC after:	2 weeks

0 files changed, 0 insertions, 0 deletions