permit mempool_free(NULL)

Christian Borntraeger points out that mempool_free() doesn't noop when
handed NULL.  This is inconsistent with the other free-like functions
in the kernel.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>
Cc: Christian Borntraeger <borntraeger@de.ibm.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

mm/mempool.c

@@ -263,6 +263,9 @@ void mempool_free(void *element, mempool_t *pool)
 {
 	unsigned long flags;
 
+	if (unlikely(element == NULL))
+		return;
+
 	smp_mb();
 	if (pool->curr_nr < pool->min_nr) {
 		spin_lock_irqsave(&pool->lock, flags);