[PATCH] sanitize unshare_files/reset_files_struct

* let unshare_files() give caller the displaced files_struct
* don't bother with grabbing reference only to drop it in the
  caller if it hadn't been shared in the first place
* in that form unshare_files() is trivially implemented via
  unshare_fd(), so we eliminate the duplicate logics in fork.c
* reset_files_struct() is not just only called for current;
  it will break the system if somebody ever calls it for anything
  else (we can't modify ->files of somebody else).  Lose the
  task_struct * argument.

Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

include/linux/fs.h

@@ -2033,9 +2033,6 @@ static inline ino_t parent_ino(struct dentry *dentry)
 	return res;
 }
 
-/* kernel/fork.c */
-extern int unshare_files(void);
-
 /* Transaction based IO helpers */
 
 /*