dm-ima: more strlen() drops

Following commit ebbd1769 ("dm: ima: avoid extra calls to
strlen()"), convert 'dm_ima_alloc_and_copy_capacity_str()' to
return the number of characters emitted by 'scnprintf()' and
simplify the users accordingly. Compile tested only.

Signed-off-by: Dmitry Antipov <dmantipov@yandex.ru>
Reviewed-by: Hou Tao <houtao1@huawei.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>