Add spl_kmem_cache_expire module option

Cache aging was implemented because it was part of the default Solaris
kmem_cache behavior.  The idea is that per-cpu objects which haven't been
accessed in several seconds should be returned to the cache.  On the other
hand Linux slabs never move objects back to the slabs unless there is
memory pressure on the system.

This behavior is now configurable through the 'spl_kmem_cache_expire'
module option.  The value is a bit mask with the following meaning.

  0x1 - Solaris style cache aging eviction is enabled.
  0x2 - Linux style low memory eviction is enabled.

Both methods may be safely enabled simultaneously, but by default
both are disabled.  It has never been clear if the kmem cache aging
(which has been around from day one) actually does any good.  It has
however been the source of numerous bugs so I wouldn't mind retiring
it entirely.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes zfsonlinux/zfs#1227
Closes #210
Define BE_IN16 & BE_IN32 for lz4 compression

The new lz4 compression algorithm, zfsonlinux/zfs@9759c60, requires
the generic BE_IN16 and BE_IN32 functions.  These are added to the SPL
for other consumers to take advantage of.

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