lib/zoap: Fix incorrect values for payload size
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 <540a5b73c8ed8ee3381dfb54c1949e767fadd754@intel.com>

finikorg/zephyr,punitvara/zephyr,rsalveti/zephyr,tidyjiang8/zephyr-doc,pklazy/zephyr,pklazy/zephyr,GiulianoFranchetto/zephyr,runchip/zephyr-cc3200,erwango/zephyr,zephyrproject-rtos/zephyr,sharronliu/zephyr,finikorg/zephyr,aceofall/zephyr-iotos,runchip/zephyr-cc3200,tidyjiang8/zephyr-doc,bboozzoo/zephyr,runchip/zephyr-cc3220,punitvara/zephyr,sharronliu/zephyr,tidyjiang8/zephyr-doc,galak/zephyr,zephyriot/zephyr,mbolivar/zephyr,punitvara/zephyr,bigdinotech/zephyr,holtmann/zephyr,ldts/zephyr,runchip/zephyr-cc3200,galak/zephyr,Vudentz/zephyr,zephyriot/zephyr,explora26/zephyr,zephyrproject-rtos/zephyr,zephyriot/zephyr,Vudentz/zephyr,zephyriot/zephyr,kraj/zephyr,zephyrproject-rtos/zephyr,erwango/zephyr,mbolivar/zephyr,sharronliu/zephyr,bigdinotech/zephyr,galak/zephyr,sharronliu/zephyr,fractalclone/zephyr-riscv,pklazy/zephyr,fbsder/zephyr,aceofall/zephyr-iotos,aceofall/zephyr-iotos,nashif/zephyr,runchip/zephyr-cc3220,Vudentz/zephyr,runchip/zephyr-cc3220,bigdinotech/zephyr,mbolivar/zephyr,zephyrproject-rtos/zephyr,runchip/zephyr-cc3200,GiulianoFranchetto/zephyr,Vudentz/zephyr,rsalveti/zephyr,mbolivar/zephyr,fbsder/zephyr,ldts/zephyr,finikorg/zephyr,pklazy/zephyr,bigdinotech/zephyr,explora26/zephyr,runchip/zephyr-cc3200,fractalclone/zephyr-riscv,pklazy/zephyr,punitvara/zephyr,fractalclone/zephyr-riscv,bboozzoo/zephyr,erwango/zephyr,holtmann/zephyr,tidyjiang8/zephyr-doc,ldts/zephyr,fractalclone/zephyr-riscv,nashif/zephyr,bboozzoo/zephyr,Vudentz/zephyr,nashif/zephyr,rsalveti/zephyr,bigdinotech/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,holtmann/zephyr,bboozzoo/zephyr,erwango/zephyr,fbsder/zephyr,kraj/zephyr,Vudentz/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,sharronliu/zephyr,aceofall/zephyr-iotos,explora26/zephyr,aceofall/zephyr-iotos,kraj/zephyr,ldts/zephyr,punitvara/zephyr,mbolivar/zephyr,runchip/zephyr-cc3220,finikorg/zephyr,kraj/zephyr,GiulianoFranchetto/zephyr,GiulianoFranchetto/zephyr,fbsder/zephyr,zephyriot/zephyr,explora26/zephyr,nashif/zephyr,fractalclone/zephyr-riscv,rsalveti/zephyr,bboozzoo/zephyr,galak/zephyr,galak/zephyr,fbsder/zephyr,runchip/zephyr-cc3220,explora26/zephyr,tidyjiang8/zephyr-doc,holtmann/zephyr,holtmann/zephyr,finikorg/zephyr,kraj/zephyr,erwango/zephyr,rsalveti/zephyr