Allocate zfs_znode_cache on the Linux slab

The Linux slab, in general, performs better than the SPl slab in cases
where a lot of objects are allocated and fragmentation is likely present.

This patch fixes pathologically bad behavior in cases where the ARC is
filled with mostly metadata and a user program needs to allocate and
dirty enough memory which would require an insignificant amount of the
ARC to be reclaimed.

If zfs_znode_cache is on the SPL slab, the system may spin for a very
long time trying to reclaim sufficient memory.  If it is on the Linux
slab, the behavior has been observed to be much more predictible; the
memory is reclaimed more efficiently.

Signed-off-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #3283

module/zfs/zfs_znode.c

@@ -147,12 +147,14 @@ void
 zfs_znode_init(void)
 {
 	/*
-	 * Initialize zcache
+	 * Initialize zcache.  The KMC_SLAB hint is used in order that it be
+	 * backed by kmalloc() when on the Linux slab in order that any
+	 * wait_on_bit() operations on the related inode operate properly.
 	 */
 	ASSERT(znode_cache == NULL);
 	znode_cache = kmem_cache_create("zfs_znode_cache",
 	    sizeof (znode_t), 0, zfs_znode_cache_constructor,
-	    zfs_znode_cache_destructor, NULL, NULL, NULL, KMC_KMEM);
+	    zfs_znode_cache_destructor, NULL, NULL, NULL, KMC_SLAB);
 }
 
 void