net: lwm2m: Fix offset for post-write callback

The offset in post-write callback is incorrect for TLV format as
the first data block contain TLV header. This fix is replacing
reamaning for offset in opaque context struct and passing it to
post-write and validation callback.

The fix also corrects the offset in pull context as it was pointing
at the end of the block processed data and not on the block context
prior to processing.

Signed-off-by: Dave Lacerte <lacerte.dave@hydroquebec.com>