Enable zfs_arc_memory_throttle_disable by default

The zfs_arc_memory_throttle_disable module option was introduced
by commit 0c5493d47059f25ce9dbf20c9fe87655f55102a1 to resolve a
memory miscalculation which could result in the txg_sync thread
spinning.

When this was first introduced the default behavior was left
unchanged until enough real world usage confirmed there were no
unexpected issues.  We've now reached that point.  Linux's
direct reclaim is working as expected so we're enabling this
behavior by default.

This helps pave the way to retire the spl_kmem_availrmem()
functionality in the SPL layer.  This was the only caller.

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #938