pstore: convert to the new mount API

Convert the pstore filesystem to the new mount API.

Cc: Kees Cook <kees@kernel.org>
Cc: Tony Luck <tony.luck@intel.com>
Signed-off-by: Eric Sandeen <sandeen@redhat.com>
Link: https://lore.kernel.org/r/20250205213931.74614-2-sandeen@redhat.com


Reviewed-by: Kees Cook <kees@kernel.org>
Signed-off-by: Christian Brauner <brauner@kernel.org>