Add vfs.zfs.reference_tracking_enable sysctl/tunable.

This is primarily for developer/debugging use; it enables built-in tagged
tracking of refcounts inside ZFS.  It can only be enabled from the loader,
since it modifies how in-core state is managed.  Default remains disabled.

MFC after:	1 week
Sponsored by:	Spectra Logic

sys/cddl/contrib/opensolaris/uts/common/fs/zfs/refcount.c

@@ -30,6 +30,10 @@
 
 #ifdef _KERNEL
 int reference_tracking_enable = FALSE; /* runs out of memory too easily */
+SYSCTL_DECL(_vfs_zfs);
+SYSCTL_INT(_vfs_zfs, OID_AUTO, reference_tracking_enable, CTLFLAG_RDTUN,
+    &reference_tracking_enable, 0,
+    "Track reference holders to refcount_t objects, used mostly by ZFS");
 #else
 int reference_tracking_enable = TRUE;
 #endif