vfs: show unreachable paths in getcwd and proc

Prepend "(unreachable)" to path strings if the path is not reachable
from the current root.

Two places updated are
 - the return string from getcwd()
 - and symlinks under /proc/$PID.

Other uses of d_path() are left unchanged (we know that some old
software crashes if /proc/mounts is changed).

Signed-off-by: Miklos Szeredi <mszeredi@suse.cz>
Signed-off-by: Al Viro <viro@zeniv.linux.org.uk>

fs/proc/base.c

@@ -1526,7 +1526,7 @@ static int do_proc_readlink(struct path *path, char __user *buffer, int buflen)
 	if (!tmp)
 		return -ENOMEM;
 
-	pathname = d_path(path, tmp, PAGE_SIZE);
+	pathname = d_path_with_unreachable(path, tmp, PAGE_SIZE);
 	len = PTR_ERR(pathname);
 	if (IS_ERR(pathname))
 		goto out;