IB/rxe: check for allocation failure on elem

The allocation for elem may fail (especially because we're using
GFP_ATOMIC) so best to check for a null return.  This fixes a potential
null pointer dereference when assigning elem->pool.

Detected by CoverityScan CID#1357507 ("Dereference null return value")

Fixes: 8700e3e7 ("Soft RoCE driver")
Signed-off-by: NColin Ian King <colin.king@canonical.com>
Signed-off-by: NDoug Ledford <dledford@redhat.com>

drivers/infiniband/sw/rxe/rxe_pool.c

@@ -404,6 +404,8 @@ void *rxe_alloc(struct rxe_pool *pool)
 	elem = kmem_cache_zalloc(pool_cache(pool),
 				 (pool->flags & RXE_POOL_ATOMIC) ?
 				 GFP_ATOMIC : GFP_KERNEL);
+	if (!elem)
+		return NULL;
 
 	elem->pool = pool;
 	kref_init(&elem->ref_cnt);