Add a NULL check for the 'bdev' parameter of
dm_verity_loadpin_is_bdev_trusted(). The function is called
by loadpin_check(), which passes the block device that
corresponds to the super block of the file system from which
a file is being loaded. Generally a super_block structure has
an associated block device, however that is not always the
case (e.g. tmpfs).

Cc: stable@vger.kernel.org # v6.0+
Fixes: b6c1c574 ("dm: Add verity helpers for LoadPin")
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Link: https://lore.kernel.org/r/20230627202800.1.Id63f7f59536d20f1ab83e1abdc1fda1471c7d031@changeidSigned-off-by: Kees Cook <keescook@chromium.org>