pstore: inode: Convert kfree() usage to __free(kfree)

Mostly as an example to myself, replace a simple allocation pattern with
the automatic kfree cleanup features now exposed by cleanup.h.

Cc: Guilherme G. Piccoli <gpiccoli@igalia.com>
Cc: Tony Luck <tony.luck@intel.com>
Cc:  <linux-hardening@vger.kernel.org>
Link: https://lore.kernel.org/r/20231205182622.1329923-1-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>