lib/zoap: Fix incorrect values for payload size

When retrieving the payload length of the payload, there was a mistake,
what was already in the buffer (buf->len) was being considered twice,
net_buf_tailroom() already takes buf->len into account, resulting in
incorrect values.

Change-Id: I3b78d55abca4a448cab5e035b9d66352a3c59110
Signed-off-by: Vinicius Costa Gomes <vinicius.gomes@intel.com>