Add kmem_cache flag for forcing kvmalloc

This adds a new KMC_KVMEM flag was added to enforce use of the
kvmalloc allocator in kmem_cache_create even for large blocks, which
may also increase performance in some specific cases (e.g. zstd), too.

Default to KVMEM instead of VMEM in spl_kmem_cache_create.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Matt Ahrens <matt@delphix.com>
Signed-off-by: Sebastian Gottschall <s.gottschall@dd-wrt.com>
Signed-off-by: Michael Niewöhner <foss@mniewoehner.de>
Closes #9034

module/os/linux/spl/spl-proc.c

@@ -662,6 +662,33 @@ static struct ctl_table spl_kmem_table[] = {
 		.mode		= 0444,
 		.proc_handler	= &proc_doslab,
 	},
+	{
+		.procname	= "slab_kvmem_total",
+		.data		= (void *)(KMC_KVMEM | KMC_TOTAL),
+		.maxlen		= sizeof (unsigned long),
+		.extra1		= &table_min,
+		.extra2		= &table_max,
+		.mode		= 0444,
+		.proc_handler	= &proc_doslab,
+	},
+	{
+		.procname	= "slab_kvmem_alloc",
+		.data		= (void *)(KMC_KVMEM | KMC_ALLOC),
+		.maxlen		= sizeof (unsigned long),
+		.extra1		= &table_min,
+		.extra2		= &table_max,
+		.mode		= 0444,
+		.proc_handler	= &proc_doslab,
+	},
+	{
+		.procname	= "slab_kvmem_max",
+		.data		= (void *)(KMC_KVMEM | KMC_MAX),
+		.maxlen		= sizeof (unsigned long),
+		.extra1		= &table_min,
+		.extra2		= &table_max,
+		.mode		= 0444,
+		.proc_handler	= &proc_doslab,
+	},
 	{},
 };