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: f8b92ba6 ("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>