mount: Don't use /proc/self to access mount points

If a process is in another pidns than /proc, the link /proc/self doesn't
work.

(00.061569) Error (mount.c:558): Can't bind-mount
46:/zdtm/live/static/tempfs.test to /tmp/cr-tmpfs.gBVwTb: No such file
or directory

But since we've switched to the mount namespace (with setns) we
can just go an open the path by its name.
Reported-by: Urgen Sherpa <urgen.sherpa@nepallink.net>
Signed-off-by: Andrey Vagin <avagin@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

mount.c

@@ -524,7 +524,6 @@ static int close_mountpoint(DIR *dfd)
 static DIR *open_mountpoint(struct mount_info *pm)
 {
 	int fd = -1, ns_old = -1;
-	char buf[PATH_MAX];
 	char mnt_path[] = "/tmp/cr-tmpfs.XXXXXX";
 
 	/*
@@ -552,8 +551,7 @@ static DIR *open_mountpoint(struct mount_info *pm)
 		goto out;
 	}
 
-	snprintf(buf, sizeof(buf), "/proc/self/root/%s", pm->mountpoint);
-	if (mount(buf, mnt_path, NULL, MS_BIND, NULL)) {
+	if (mount(pm->mountpoint, mnt_path, NULL, MS_BIND, NULL)) {
 		pr_perror("Can't bind-mount %d:%s to %s",
 				pm->mnt_id, pm->mountpoint, mnt_path);
 		rmdir(mnt_path);