net: lwm2m: Fix OMA TLV objlnk encoding

At some point OMA TLV integer encoding was optimised to use the smallest
size possible. This broke the objlnk encoding, which internally used
`put_s32()`, but should always be 4 bytes long. In case the result
32-bit integer would fit into 16 or 8 bytes, it'd get optimized.

Fix this, by creating and writing TLV manually for objlnk instead of
relying on int32 encoder.

Signed-off-by: Robert Lubos <robert.lubos@nordicsemi.no>