pstore: inode: Convert mutex usage to guard(mutex)

Replace open-coded mutex handling with cleanup.h guard(mutex) and
scoped_guard(mutex, ...).

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/20231205182622.1329923-2-keescook@chromium.org
Signed-off-by: Kees Cook <keescook@chromium.org>