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>

include/linux/path.h

@@ -12,4 +12,9 @@ struct path {
 extern void path_get(struct path *);
 extern void path_put(struct path *);
 
+static inline int path_equal(const struct path *path1, const struct path *path2)
+{
+	return path1->mnt == path2->mnt && path1->dentry == path2->dentry;
+}
+
 #endif  /* _LINUX_PATH_H */