sunrpc: fix crash in rpc_malloc()

While the comment says:
 * To prevent rpciod from hanging, this allocator never sleeps,
 * returning NULL if the request cannot be serviced immediately.

The function does not actually check for NULL pointers being returned.

Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
Signed-off-by: Trond Myklebust <Trond.Myklebust@netapp.com>