dm: verity-loadpin: Drop use of dm_table_get_num_targets()

Commit 2aec377a ("dm table: remove dm_table_get_num_targets()
wrapper") in linux-dm/for-next removed the function
dm_table_get_num_targets() which is used by verity-loadpin. Access
table->num_targets directly instead of using the defunct wrapper.

Fixes: b6c1c574 ("dm: Add verity helpers for LoadPin")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Reviewed-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Kees Cook <keescook@chromium.org>
Link: https://lore.kernel.org/r/20220728085412.1.I242d21b378410eb6f9897a3160efb56e5608c59d@changeid