RDMA/amso1100: Add check if cache memory was allocated before freeing it

There is a path in handle_vq() where kmem_cache_free() can be called
with pointer to a local variable.  It can happen if vq_repbuf_alloc()
failed to allocate memory from cache and req is NULL.

The patch adds check if cache memory was allocated before freeing it.

Found by Linux Driver Verification project (linuxtesting.org).

Signed-off-by: Svetlana Mavrina <another.karnil@gmail.com>
Reviewed-by: Alexey Khoroshilov <khoroshilov@ispras.ru>
Signed-off-by: Roland Dreier <roland@purestorage.com>