Integrate ARC more tightly with Linux

Under Solaris the ARC was designed to stay one step ahead of the
VM subsystem. It would attempt to recognize low memory situtions
before they occured and evict data from the cache. It would also
make assessments about if there was enough free memory to perform
a specific operation.

This was all possible because Solaris exposes a fairly decent
view of the memory state of the system to other kernel threads.
Linux on the other hand does not make this information easily
available. To avoid extensive modifications to the ARC the SPL
attempts to provide these same interfaces. While this works it
is not ideal and problems can arise when the ARC and Linux have
different ideas about when your out of memory. This has manifested
itself in the past as a spinning arc_reclaim_thread.

This patch abandons the emulated Solaris interfaces in favor of
the prefered Linux interface. That means moving the bulk of the
memory reclaim logic out of the arc_reclaim_thread and in to the
evict driven shrinker callback. The Linux VM will call this
function when it needs memory. The ARC is then responsible for
attempting to free the requested amount of memory if possible.

Several interfaces have been modified to accomidate this approach,
however the basic user space implementation remains the same.
The following changes almost exclusively just apply to the kernel
implementation.

• Removed the hdr_recl() reclaim callback which is redundant with the broader arc_shrinker_func().

• Reduced arc_grow_retry to 5 seconds from 60. This is now used internally in the ARC with arc_no_grow to indicate that direct reclaim was recently performed. This typically indicates a rapid change in memory demands which the kswapd threads were unable to keep ahead of. As long as direct reclaim is happening once every 5 seconds arc growth will be paused to avoid further contributing to the existing memory pressure. The more common indirect reclaim paths will not set arc_no_grow.

• arc_shrink() has been extended to take the number of bytes by which arc_c should be reduced. This allows for a more granual reduction of the arc target. Since the kernel provides a reclaim value to the arc_shrinker_func() this value is used instead of 1<<arc_shrink_shift.

• arc_reclaim_needed() has been removed. It was used to determine if the system was under memory pressure and relied extensively on Solaris specific VM interfaces. In most case the new code just checks arc_no_grow which indicates that within the last arc_grow_retry seconds direct memory reclaim occurred.

• arc_memory_throttle() has been updated to always include the amount of evictable memory (arc and page cache) in its free space calculations. This space is largely available in most call paths due to direct memory reclaim.

• The Solaris pageout code was also removed to avoid confusion. It has always been disabled due to proc_pageout being defined as NULL in the Linux port.

Signed-off-by: Prakash Surya <surya1@llnl.gov>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>