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

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

[ Upstream commit e2eeddefb046dbc771a6fa426f7f98fb25adfe68 ]

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>
Stable-dep-of: a43e0fc5e913 ("pstore: inode: Only d_invalidate() is needed")
Signed-off-by: Sasha Levin <sashal@kernel.org>