net/context: Lock/unlock mutex on new send/sendto functions
net/context: Lock/unlock mutex on new send/sendto functions

This was forgotten modification as commit 93e5181fbdf came in master
though commit 0d519f7bcf7 was already written.

Thus fixing the missing lock/unlock.

Signed-off-by: Tomasz Bursztyka <ba81a3a719836727e6857ae83462b9f52ec41006@linux.intel.com>

Vudentz/zephyr,ldts/zephyr,Vudentz/zephyr,Vudentz/zephyr,galak/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,nashif/zephyr,Vudentz/zephyr,galak/zephyr,galak/zephyr,finikorg/zephyr,explora26/zephyr,nashif/zephyr,finikorg/zephyr,zephyrproject-rtos/zephyr,ldts/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,nashif/zephyr,GiulianoFranchetto/zephyr,galak/zephyr,explora26/zephyr,ldts/zephyr,zephyrproject-rtos/zephyr,explora26/zephyr,ldts/zephyr,galak/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,zephyrproject-rtos/zephyr,finikorg/zephyr,GiulianoFranchetto/zephyr,Vudentz/zephyr,Vudentz/zephyr,finikorg/zephyr,nashif/zephyr,nashif/zephyr,explora26/zephyr