file: fix falloc_noinstall() error case

'new file' doesn't return nullptr, it throws (well except in osv where new
always succeeds).

Replace with new (nothrow) to get the expected behaviour.

Signed-off-by: Avi Kivity <avi@cloudius-systems.com>

fs/vfs/kern_descrip.cc

@@ -159,7 +159,7 @@ static int falloc_noinstall(struct file **resultfp)
 {
     struct file *fp;
 
-    fp = new file;
+    fp = new (std::nothrow) file;
     if (!fp)
         return ENOMEM;