execute: Drop log level to unit log level in exec_spawn()

execute: Drop log level to unit log level in exec_spawn()

All messages logged from exec_spawn() are attributed to the unit
and as such we should set the log level to the unit's max log level
for the duration of the function.

(cherry picked from commit 7881f485c9f57b1c7de4308eeab54458890c5c19)