Clean up OS-specific ARC and kmem code

OS-specific code (e.g. under `module/os/linux`) does not need to share
its code structure with any other operating systems.  In particular, the
ARC and kmem code need not be similar to the code in illumos, because we
won't be syncing this OS-specific code between operating systems.  For
example, if/when illumos support is added to the common repo, we would
add a file `module/os/illumos/zfs/arc_os.c` for the illumos versions of
this code.

Therefore, we can simplify the code in the OS-specific ARC and kmem
routines.

These changes do not impact system behavior, they are purely code
cleanup.  The changes are:

Arenas are not used on Linux or FreeBSD (they are always `NULL`), so
`heap_arena`, `zio_arena`, and `zio_alloc_arena` can be removed, along
with code that uses them.

In `arc_available_memory()`:
 * `desfree` is unused, remove it
 * rename `freemem` to avoid conflict with pre-existing `#define`
 * remove checks related to arenas
 * use units of bytes, rather than converting from bytes to pages and
   then back to bytes

`SPL_KMEM_CACHE_REAP` is unused, remove it.

`skc_reap` is unused, remove it.

The `count` argument to `spl_kmem_cache_reap_now()` is unused, remove
it.

`vmem_size()` and associated type and macros are unused, remove them.

In `arc_memory_throttle()`, use a less confusing variable name to store
the result of `arc_free_memory()`.

Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@ixsystems.com>
Signed-off-by: Matthew Ahrens <mahrens@delphix.com>
Closes #10499

module/zfs/arc.c

@@ -390,11 +390,6 @@ static boolean_t arc_initialized;
  */
 boolean_t arc_warm;
 
-/*
- * log2 fraction of the zio arena to keep free.
- */
-int arc_zio_arena_free_shift = 2;
-
 /*
  * These tunables are for performance analysis.
  */
@@ -4687,14 +4682,6 @@ arc_kmem_reap_soon(void)
 	kmem_cache_reap_now(hdr_l2only_cache);
 	kmem_cache_reap_now(zfs_btree_leaf_cache);
 	abd_cache_reap_now();
-
-	if (zio_arena != NULL) {
-		/*
-		 * Ask the vmem arena to reclaim unused memory from its
-		 * quantum caches.
-		 */
-		vmem_qcache_reap(zio_arena);
-	}
 }
 
 /* ARGSUSED */