tree-wide: add DEBUG_LOGGING macro that checks whether debug logging is on (#7645)

This makes things a bit easier to read I think, and also makes sure we
always use the _unlikely_ wrapper around it, which so far we used
sometimes and other times we didn't. Let's clean that up.

src/shared/logs-show.c

@@ -1392,7 +1392,7 @@ int show_journal_by_unit(
         if (r < 0)
                 return log_error_errno(r, "Failed to add unit matches: %m");
 
-        if (_unlikely_(log_get_max_level() >= LOG_DEBUG)) {
+        if (DEBUG_LOGGING) {
                 _cleanup_free_ char *filter;
 
                 filter = journal_make_match_string(j);

src/shared/seccomp-util.c

@@ -954,7 +954,7 @@ int seccomp_restrict_namespaces(unsigned long retain) {
         uint32_t arch;
         int r;
 
-        if (log_get_max_level() >= LOG_DEBUG) {
+        if (DEBUG_LOGGING) {
                 _cleanup_free_ char *s = NULL;
 
                 (void) namespace_flag_to_string_many(retain, &s);