SUNRPC: Fix a nfs4 over rdma transport oops

Prevent an RPC oops when freeing a dynamically allocated RDMA
buffer, used in certain special-case large metadata operations.

Signed-off-by: Tom Talpey <tmt@netapp.com>
Signed-off-by: James Lentini <jlentini@netapp.com>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>