new helper: alloc_file_clone()

alloc_file_clone(old_file, mode, ops): create a new struct file with
->f_path equal to that of old_file.  pipe converted.

Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/file.h

@@ -23,6 +23,8 @@ extern struct file *alloc_file(const struct path *, int flags,
 	const struct file_operations *fop);
 extern struct file *alloc_file_pseudo(struct inode *, struct vfsmount *,
 	const char *, int flags, const struct file_operations *);
+extern struct file *alloc_file_clone(struct file *, int flags,
+	const struct file_operations *);
 
 static inline void fput_light(struct file *file, int fput_needed)
 {