file: make fo_init() optional

Derived file objects will be initialized by the class constructor, no need
for fo_init().

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

fs/vfs/kern_descrip.cc

@@ -163,7 +163,9 @@ file::file(unsigned flags, filetype_t type, void *opaque,
     auto fp = this;
     TAILQ_INIT(&fp->f_poll_list);
 
-    fo_init(fp);
+    if (ops) {
+        fo_init(fp);
+    }
 }
 
 int file::read(struct uio *uio, int flags)

include/osv/file.h

@@ -75,7 +75,7 @@ struct file;
  */
 struct file {
 	file(unsigned flags, filetype_t type, void *opaque,
-		struct fileops *ops);
+		struct fileops *ops = nullptr);
 	virtual ~file();
 	void operator delete(void *p) { osv::rcu_dispose(p); }