sysctl: consider --prefix while parsing the files

not while applying the parsed sysctl values. Otherwise
info "Overwriting earlier assignment of %s in file %s" is
visible many times even though the given --prefix doesn't
try to set the overridden value.

This also optimizes the startup tiny bit since we have udev
rules running on network devices and setting sysctl through
the rules.

(cherry picked from commit b99802f769)

src/sysctl/sysctl.c

@@ -86,22 +86,6 @@ static int apply_sysctl(const char *property, const char *value) {
         n = stpcpy(p, "/proc/sys/");
         strcpy(n, property);
 
-        if (!strv_isempty(arg_prefixes)) {
-                char **i;
-                bool good = false;
-
-                STRV_FOREACH(i, arg_prefixes)
-                        if (path_startswith(p, *i)) {
-                                good = true;
-                                break;
-                        }
-
-                if (!good) {
-                        log_debug("Skipping %s", p);
-                        return 0;
-                }
-        }
-
         k = write_string_file(p, value);
         if (k < 0) {
                 log_full(k == -ENOENT ? LOG_DEBUG : LOG_WARNING,
@@ -182,6 +166,20 @@ static int parse_file(Hashmap *sysctl_options, const char *path, bool ignore_eno
                 p = normalize_sysctl(strstrip(p));
                 value = strstrip(value);
 
+                if (!strv_isempty(arg_prefixes)) {
+                        char **i, *t;
+                        STRV_FOREACH(i, arg_prefixes) {
+                                t = path_startswith(*i, "/proc/sys/");
+                                if (t == NULL)
+                                        t = *i;
+                                if (path_startswith(p, t))
+                                        goto found;
+                        }
+                        /* not found */
+                        continue;
+                }
+
+found:
                 existing = hashmap_get2(sysctl_options, p, &v);
                 if (existing) {
                         if (streq(value, existing))