net: lib: lwm2m: block 0 resets prev block transfer for same resource

As stated in "[RFC7959], Section 2.5":
   "The Block1 Option provides no way for a single endpoint to perform
   multiple concurrently proceeding block-wise request payload transfer
   (e.g., PUT or POST) operations to the same resource.  Starting a new
   block-wise sequence of requests to the same resource (before an old
   sequence from the same endpoint was finished) simply overwrites the
   context the server may still be keeping."

Signed-off-by: Marco Argiolas <marco.argiolas@ftpsolutions.com.au>