NFSD: Correct the size calculation in fault_inject_write

If len == 0 we end up with size = (0 - 1), which could cause bad things
to happen in copy_from_user().
Signed-off-by: Bryan Schumaker <bjschuma@netapp.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>

fs/nfsd/fault_inject.c

@@ -122,7 +122,7 @@ static ssize_t fault_inject_write(struct file *file, const char __user *buf,
 				  size_t len, loff_t *ppos)
 {
 	char write_buf[INET6_ADDRSTRLEN];
-	size_t size = min(sizeof(write_buf), len) - 1;
+	size_t size = min(sizeof(write_buf) - 1, len);
 	struct net *net = current->nsproxy->net_ns;
 	struct sockaddr_storage sa;
 	u64 val;