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

include/sys/arc_impl.h

@@ -872,7 +872,6 @@ typedef enum free_memory_reason_t {
 #define	arc_sys_free	ARCSTAT(arcstat_sys_free) /* target system free bytes */
 #define	arc_need_free	ARCSTAT(arcstat_need_free) /* bytes to be freed */
 
-extern int arc_zio_arena_free_shift;
 extern taskq_t *arc_prune_taskq;
 extern arc_stats_t arc_stats;
 extern hrtime_t arc_growtime;

include/sys/zfs_context.h

@@ -413,12 +413,9 @@ void procfs_list_add(procfs_list_t *procfs_list, void *p);
 #define	kmem_debugging()	0
 #define	kmem_cache_reap_now(_c)	umem_cache_reap_now(_c);
 #define	kmem_cache_set_move(_c, _cb)	/* nothing */
-#define	vmem_qcache_reap(_v)		/* nothing */
 #define	POINTER_INVALIDATE(_pp)		/* nothing */
 #define	POINTER_IS_VALID(_p)	0
 
-extern vmem_t *zio_arena;
-
 typedef umem_cache_t kmem_cache_t;
 
 typedef enum kmem_cbrc {