Fix bug introduced in rewrite of keg_free_slab in -r251894.

The consequence of the bug is that fini calls are not done
when a slab is freed by a call-back from the page daemon.
It went unnoticed for two months because fini is little used.

I spotted the bug while reading the code to learn how it works
so I could write it up for the next edition of the Design and
Implementation of FreeBSD book.

No MFC needed as this code exists only in HEAD.

Reviewed by: kib, jeff
Tested by:   pho

sys/vm/uma_core.c

@@ -780,7 +780,7 @@ keg_drain(uma_keg_t keg)
 
 	while ((slab = SLIST_FIRST(&freeslabs)) != NULL) {
 		SLIST_REMOVE(&freeslabs, slab, uma_slab, us_hlink);
-		keg_free_slab(keg, slab, 0);
+		keg_free_slab(keg, slab, keg->uk_ipers);
 	}
 }