RDMA/srpt: Use kmem_cache_free() instead of kfree()

memory allocated by kmem_cache_alloc() should be freed using
kmem_cache_free(), not kfree().

Fixes: 5dabcd04 ("RDMA/srpt: Add support for immediate data")
Signed-off-by: Wei Yongjun <weiyongjun1@huawei.com>
Signed-off-by: Jason Gunthorpe <jgg@mellanox.com>