Linux: Set spl_kmem_cache_slab_limit when page size !4K

For small objects the kernel's slab implementation is very fast and
space efficient. However, as the allocation size increases to
require multiple pages performance suffers. The SPL kmem cache
allocator was designed to better handle these large allocation
sizes. Therefore, on Linux the kmem_cache_* compatibility wrappers
prefer to use the kernel's slab allocator for small objects and
the custom SPL kmem cache allocator for larger objects.

This logic was effectively disabled for all architectures using
a non-4K page size which caused all kmem caches to only use the
SPL implementation. Functionally this is fine, but the SPL code
which calculates the target number of objects per-slab does not
take in to account that __vmalloc() always returns page-aligned
memory. This can result in a massive amount of wasted space when
allocating tiny objects on a platform using large pages (64k).

To resolve this issue we set the spl_kmem_cache_slab_limit cutoff
to 16K for all architectures. 

This particular change does not attempt to update the logic used
to calculate the optimal number of pages per slab. This remains
an issue which should be addressed in a future change.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Tony Nguyen <tony.nguyen@delphix.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes openzfs#12152
Closes openzfs#11429
Closes openzfs#11574
Closes openzfs#12150

module/os/linux/spl/spl-kmem-cache.c

@@ -100,13 +100,10 @@ MODULE_PARM_DESC(spl_kmem_cache_max_size, "Maximum size of slab in MB");
  * For small objects the Linux slab allocator should be used to make the most
  * efficient use of the memory.  However, large objects are not supported by
  * the Linux slab and therefore the SPL implementation is preferred.  A cutoff
- * of 16K was determined to be optimal for architectures using 4K pages.
+ * of 16K was determined to be optimal for architectures using 4K pages and
+ * to also work well on architecutres using larger 64K page sizes.
  */
-#if PAGE_SIZE == 4096
 unsigned int spl_kmem_cache_slab_limit = 16384;
-#else
-unsigned int spl_kmem_cache_slab_limit = 0;
-#endif
 module_param(spl_kmem_cache_slab_limit, uint, 0644);
 MODULE_PARM_DESC(spl_kmem_cache_slab_limit,
 	"Objects less than N bytes use the Linux slab");