If the freed block is smaller than a KernFree, then just discard it

Do this rather than trying to insert back into the freelist, because we
use the free blocks to store the freelist links, and trying to write
into this results in overwriting adjacent memory when the freed block is
too small.

src/allocator.rs

@@ -449,6 +449,11 @@ impl InnerKernelAlloc {
                         let b = *a;
                         *a = KernAllocation::default();
 
+                        if b.length < core::mem::size_of::<KernFree>() {
+                            trace!("Freed block (len={}) less than KernFree (len={})", b.length, core::mem::size_of::<KernFree>());
+                            return;
+                        }
+
                         // Then add it back to the free list
                         if let Some(fl) = (*s).freelist {
                             // If it belongs at the head, insert it at the head