arc_os: split userspace and Linux kernel code

The Linux arc_os.c carries userspace and kernel code, with very little
overlap between the two. This lifts the userspace parts out into a
separate arc_os.c for libzpool and removes it from the Linux side.

Reviewed by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Signed-off-by: Rob Norris <robn@despairlabs.com>
Sponsored-by: https://despairlabs.com/sponsor/
Closes #16492

module/os/linux/zfs/arc_os.c

@@ -42,14 +42,12 @@
 #include <sys/abd.h>
 #include <sys/zil.h>
 #include <sys/fm/fs/zfs.h>
-#ifdef _KERNEL
 #include <sys/shrinker.h>
 #include <sys/vmsystm.h>
 #include <sys/zpl.h>
 #include <linux/page_compat.h>
 #include <linux/notifier.h>
 #include <linux/memory.h>
-#endif
 #include <sys/callb.h>
 #include <sys/kstat.h>
 #include <sys/zthr.h>
@@ -88,7 +86,6 @@ arc_default_max(uint64_t min, uint64_t allmem)
 	return (MAX(allmem / 2, min));
 }
 
-#ifdef _KERNEL
 /*
  * Return maximum amount of memory that we could possibly use.  Reduced
  * to half of all memory in user space which is primarily used for testing.
@@ -449,48 +446,6 @@ arc_unregister_hotplug(void)
 	unregister_memory_notifier(&arc_hotplug_callback_mem_nb);
 #endif
 }
-#else /* _KERNEL */
-int64_t
-arc_available_memory(void)
-{
-	int64_t lowest = INT64_MAX;
-
-	/* Every 100 calls, free a small amount */
-	if (random_in_range(100) == 0)
-		lowest = -1024;
-
-	return (lowest);
-}
-
-int
-arc_memory_throttle(spa_t *spa, uint64_t reserve, uint64_t txg)
-{
-	(void) spa, (void) reserve, (void) txg;
-	return (0);
-}
-
-uint64_t
-arc_all_memory(void)
-{
-	return (ptob(physmem) / 2);
-}
-
-uint64_t
-arc_free_memory(void)
-{
-	return (random_in_range(arc_all_memory() * 20 / 100));
-}
-
-void
-arc_register_hotplug(void)
-{
-}
-
-void
-arc_unregister_hotplug(void)
-{
-}
-#endif /* _KERNEL */
 
 ZFS_MODULE_PARAM(zfs_arc, zfs_arc_, shrinker_limit, INT, ZMOD_RW,
 	"Limit on number of pages that ARC shrinker can reclaim at once");