allow the temp files created by open() to be linked to

O_TMPFILE | O_CREAT => linkat() with AT_SYMLINK_FOLLOW and /proc/self/fd/<n>
as oldpath (i.e. flink()) will create a link
O_TMPFILE | O_CREAT | O_EXCL => ENOENT on attempt to link those guys

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

fs/inode.c

@@ -333,8 +333,10 @@ EXPORT_SYMBOL(set_nlink);
  */
 void inc_nlink(struct inode *inode)
 {
-	if (WARN_ON(inode->i_nlink == 0))
+	if (unlikely(inode->i_nlink == 0)) {
+		WARN_ON(!(inode->i_state & I_LINKABLE));
 		atomic_long_dec(&inode->i_sb->s_remove_count);
+	}
 
 	inode->__i_nlink++;
 }