mount: handle OOM on mnt_warn_timestamp_expiry

If no page could be allocated, an error pointer was used as format
string in pr_warn.

Rearrange the code to return early in case of OOM. Also add a check
for the return value of d_path.

Fixes: f8b92ba ("mount: Add mount warning for impending timestamp expiry")
Signed-off-by: Olaf Hering <olaf@aepfle.de>
Link: https://lore.kernel.org/r/20240730085856.32385-1-olaf@aepfle.de
[brauner: rewrite commit and commit message]
Signed-off-by: Christian Brauner <brauner@kernel.org>

fs/namespace.c

@@ -2921,17 +2921,25 @@ static void mnt_warn_timestamp_expiry(struct path *mountpoint, struct vfsmount *
 	if (!__mnt_is_readonly(mnt) &&
 	   (!(sb->s_iflags & SB_I_TS_EXPIRY_WARNED)) &&
 	   (ktime_get_real_seconds() + TIME_UPTIME_SEC_MAX > sb->s_time_max)) {
-		char *buf = (char *)__get_free_page(GFP_KERNEL);
-		char *mntpath = buf ? d_path(mountpoint, buf, PAGE_SIZE) : ERR_PTR(-ENOMEM);
+		char *buf, *mntpath;
+
+		buf = (char *)__get_free_page(GFP_KERNEL);
+		if (buf)
+			mntpath = d_path(mountpoint, buf, PAGE_SIZE);
+		else
+			mntpath = ERR_PTR(-ENOMEM);
+		if (IS_ERR(mntpath))
+			mntpath = "(unknown)";
 
 		pr_warn("%s filesystem being %s at %s supports timestamps until %ptTd (0x%llx)\n",
 			sb->s_type->name,
 			is_mounted(mnt) ? "remounted" : "mounted",
 			mntpath, &sb->s_time_max,
 			(unsigned long long)sb->s_time_max);
 
-		free_page((unsigned long)buf);
 		sb->s_iflags |= SB_I_TS_EXPIRY_WARNED;
+		if (buf)
+			free_page((unsigned long)buf);
 	}
 }