kernel: nfs: Handle error of rpc_proc_register() in nfs_net_init().

In the Linux kernel, the following vulnerability has been resolved: nfs: Handle error of rpcprocregister in nfsnetinit. syzkaller reported a warning 0 triggered while destroying immature netns. rpcprocregister was called in initnfsfs, but its error has been ignored since at least the initial comm...