With this change, whether something should be logged or, that is,
whether the used log priority is enabled or not, is checked before
the logging function is called. That should save a few cycles.
Now mxs_log_message() always logs a message, without consideration
for whether the priority is enabled or not. The inline function
mxs_log_is_priority_enabled() returns true if a particular priority
is enabled and the MXS_LOG_MESSAGE() macro (that MXS_ERROR etc. are
expanded into) call that before calling the actual logging function.
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