abd_os: break out platform-specific header parts

Removing the platform #ifdefs from shared headers in favour of
per-platform headers. Makes abd_t much leaner, among other things.

Sponsored-by: Klara, Inc.
Sponsored-by: Wasabi Technology, Inc.
Reviewed-by: Alexander Motin <mav@FreeBSD.org>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rob Norris <rob.norris@klarasystems.com>
Closes #16253

module/os/freebsd/zfs/abd_os.c

@@ -95,14 +95,12 @@ struct {
  */
 static size_t zfs_abd_scatter_min_size = PAGE_SIZE + 1;
 
-#if defined(_KERNEL)
 SYSCTL_DECL(_vfs_zfs);
 
 SYSCTL_INT(_vfs_zfs, OID_AUTO, abd_scatter_enabled, CTLFLAG_RWTUN,
 	&zfs_abd_scatter_enabled, 0, "Enable scattered ARC data buffers");
 SYSCTL_ULONG(_vfs_zfs, OID_AUTO, abd_scatter_min_size, CTLFLAG_RWTUN,
 	&zfs_abd_scatter_min_size, 0, "Minimum size of scatter allocations.");
-#endif
 
 kmem_cache_t *abd_chunk_cache;
 static kstat_t *abd_ksp;