proc_fd_permission() is doesn't need to bail out in RCU mode

nothing blocking except generic_permission()
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/proc/base.c

@@ -2169,11 +2169,7 @@ static const struct file_operations proc_fd_operations = {
  */
 static int proc_fd_permission(struct inode *inode, int mask, unsigned int flags)
 {
-	int rv;
-
-	if (flags & IPERM_FLAG_RCU)
-		return -ECHILD;
-	rv = generic_permission(inode, mask, flags, NULL);
+	int rv = generic_permission(inode, mask, flags, NULL);
 	if (rv == 0)
 		return 0;
 	if (task_pid(current) == proc_pid(inode))