Linux 3.1 compat, super_block->s_shrink

The Linux 3.1 kernel has introduced the concept of per-filesystem
shrinkers which are directly assoicated with a super block. Prior
to this change there was one shared global shrinker.

The zfs code relied on being able to call the global shrinker when
the arc_meta_limit was exceeded. This would cause the VFS to drop
references on a fraction of the dentries in the dcache. The ARC
could then safely reclaim the memory used by these entries and
honor the arc_meta_limit. Unfortunately, when per-filesystem
shrinkers were added the old interfaces were made unavailable.

This change adds support to use the new per-filesystem shrinker
interface so we can continue to honor the arc_meta_limit. The
major benefit of the new interface is that we can now target
only the zfs filesystem for dentry and inode pruning. Thus we
can minimize any impact on the caching of other filesystems.

In the context of making this change several other important
issues related to managing the ARC were addressed, they include:

• The dnlc_reduce_cache() function which was called by the ARC

to drop dentries for the Posix layer was replaced with a generic
zfs_prune_t callback. The ZPL layer now registers a callback to
drop these dentries removing a layering violation which dates
back to the Solaris code. This callback can also be used by
other ARC consumers such as Lustre.

arc_add_prune_callback()
arc_remove_prune_callback()

• The arc_reduce_dnlc_percent module option has been changed to

arc_meta_prune for clarity. The dnlc functions are specific to
Solaris's VFS and have already been largely eliminated already.
The replacement tunable now represents the number of bytes the
prune callback will request when invoked.

• Less aggressively invoke the prune callback. We used to call

this whenever we exceeded the arc_meta_limit however that's not
strictly correct since it results in over zeleous reclaim of
dentries and inodes. It is now only called once the arc_meta_limit
is exceeded and every effort has been made to evict other data from
the ARC cache.

• More promptly manage exceeding the arc_meta_limit. When reading

meta data in to the cache if a buffer was unable to be recycled
notify the arc_reclaim thread to invoke the required prune.

• Added arcstat_prune kstat which is incremented when the ARC

is forced to request that a consumer prune its cache. Remember
this will only occur when the ARC has no other choice. If it
can evict buffers safely without invoking the prune callback
it will.

• This change is also expected to resolve the unexpect collapses

of the ARC cache. This would occur because when exceeded just the
arc_meta_limit reclaim presure would be excerted on the arc_c
value via arc_shrink(). This effectively shrunk the entire cache
when really we just needed to reclaim meta data.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #466
Closes #292