parlib: slab: Fix ancient ctor off-by-one

The original purpose for that list was to build the chain of small slab
objects.  But we only ran the ctor on the first n - 1 of them.

This bug is ancient - it's from the kernel's original slab implementation.
We hadn't really used slab ctors a lot.  Same goes for userspace, until my
recent epoll changes.

Signed-off-by: Barret Rhoden <brho@cs.berkeley.edu>

user/parlib/slab.c

diff --git a/user/parlib/slab.c b/user/parlib/slab.c
index f65c263..f094c11 100644
--- a/user/parlib/slab.c
+++ b/user/parlib/slab.c
@@ -274,6 +274,9 @@
 			*(uintptr_t**)(buf + cp->obj_size) = buf + a_slab->obj_size;
 			buf += a_slab->obj_size;
 		}
+		/* Initialize the final object (note the -1 in the for loop). */
+		if (cp->ctor)
+			cp->ctor(buf, cp->obj_size);
 		*((uintptr_t**)(buf + cp->obj_size)) = NULL;
 	} else {
 		a_slab = kmem_cache_alloc(kmem_slab_cache, 0);