nfs: Fix oops in nfs_netfs_init_request() when copying to cache

In the Linux kernel, the following vulnerability has been resolved: nfs: Fix oops in nfs_netfs_init_request() when copying to cache When netfslib wants to copy some data that has just been read on behalf of nfs, it creates a new write request and calls nfs_netfs_init_request() to initialise it, but with a NULL file pointer. This causes nfs_file_open_context() to oops - however, we don't actually need the nfs context as we're only going to write to the cache. Fix this by just returning if we aren't given a file pointer and emit a warning if the request was for something other than copy-to-cache. Further, fix nfs_netfs_free_request() so that it doesn't try to free the context if the pointer is NULL.