TRIM cache devices based on time instead of TXGs.
Currently, the trim module uses the same algorithm for data and cache
devices when deciding to issue TRIM requests, based on how far in the
past the TXG is.

Unfortunately, this is not ideal for cache devices, because the L2ARC
doesn't use the concept of TXGs at all. In fact, when using a pool for
reading only, the L2ARC is written but the TXG counter doesn't
increase, and so no new TRIM requests are issued to the cache device.

This patch fixes the issue by using time instead of the TXG number as
the criteria for trimming on cache devices. The basic delay principle
stays the same, but parameters are expressed in seconds instead of
TXGs. The new parameters are named trim_l2arc_limit and
trim_l2arc_batch, and both default to 30 second.

Reviewed by:	pjd (mentor)
Approved by:	pjd (mentor)
Obtained from:	https://github.com/dechamps/zfs/commit/17122c31ac7f82875e837019205c21651c05f8cd
MFC after:	2 weeks
Names the ZFS TRIM thread

Reviewed by:	pjd (mentor)
Approved by:	pjd (mentor)
MFC after:	2 weeks
Optimisation of TRIM processing.

Previously TRIM processing was very bursty. This was made worse by the fact
that TRIM requests on SSD's are typically much slower than reads or writes.
This often resulted in stalls while large numbers of TRIM's where processed.

In addition due to the way the TRIM thread was only woken by writes, deletes
could stall in the queue for extensive periods of time.

This patch adds a number of controls to how often the TRIM thread for each
SPA processes its outstanding delete requests.
vfs.zfs.trim.timeout: Delay TRIMs by up to this many seconds
vfs.zfs.trim.txg_delay: Delay TRIMs by up to this many TXGs (reduced to 32)
vfs.zfs.vdev.trim_max_bytes: Maximum pending TRIM bytes for a vdev
vfs.zfs.vdev.trim_max_pending: Maximum pending TRIM segments for a vdev
vfs.zfs.trim.max_interval: Maximum interval between TRIM queue processing
(seconds)

Given the most common TRIM implementation is ATA TRIM the current defaults
are targeted at that.

Reviewed by:	pjd (mentor)
Approved by:	pjd (mentor)
MFC after:	2 weeks
Remove EOL whitespace.
Add missing descriptions for ZFS sysctls

Reviewed by:	pjd (mentor)
Approved by:	pjd (mentor)
MFC after:	2 weeks