Add TRIM support for L2ARC.

This adds TRIM support to cache vdevs. When ARC buffers are removed
from the L2ARC in arc_hdr_destroy(), arc_release() or l2arc_evict(),
the size previously occupied by the buffer gets scheduled for TRIMming.
As always, actual TRIMs are only issued to the L2ARC after
txg_trim_limit.

Reviewed by:	pjd (mentor)
Approved by:	pjd (mentor)
Obtained from:	https://github.com/dechamps/zfs/commit/31aae373994fd112256607edba7de2359da3e9dc
MFC after:	2 weeks
Don't register repair writes in the trim map.

The trim map inflight writes tree assumes non-conflicting writes, i.e.
that there will never be two simultaneous write I/Os to the same range
on the same vdev. This seemed like a sane assumption; however, in
actual testing, it appears that repair I/Os can very well conflict
with "normal" writes.

I'm not quite sure if these conflicting writes are supposed to happen
or not, but in the mean time, let's ignore repair writes for now. This
should be safe considering that, by definition, we never repair blocks
that are freed.

Reviewed by:	pjd (mentor)
Approved by:	pjd (mentor)
Obtained from:	Source: https://github.com/dechamps/zfs/commit/6a3cebaf7c5fcc92007280b5d403c15d0e61dfe3
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   current SPA syncing TXG (which may be out of date);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