dm cache: drop redundant origin size check

dm cache: drop redundant origin size check

The cache target already exposes the origin device through
cache_iterate_devices(), which allows dm-table to call
device_area_is_invalid() and verify that the mapping fits inside the
underlying block device.

The explicit ti->len > origin_sectors test in parse_origin_dev() is
therefore redundant. Drop this check and rely on the core device
validation instead. This changes the user-visible error string when the
origin is too small, but preserves the failure behaviour.

Signed-off-by: Li Chen <me@linux.beauty>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>