core: add log_get_max_level check optimization in log_unit_full

core: add log_get_max_level check optimization in log_unit_full

Just as log_full already does, check if the log level would result in
logging immediately in the macro in order to avoid doing
unnecessary work that adds up in hot spots.

(cherry picked from commit c5bc2c01eef4d3defb58245960065a063f5f75cf)