sysctl: disable buffer while writing to /proc

fputs() writes only first 2048 bytes and fails
to write to /proc when values are larger than that.
This patch adds a new flag to WriteStringFileFlags
that make it possible to disable the buffer under
specific cases.

src/shared/sysctl-util.c

@@ -60,7 +60,7 @@ int sysctl_write(const char *property, const char *value) {
         log_debug("Setting '%s' to '%s'", property, value);
 
         p = strjoina("/proc/sys/", property);
-        return write_string_file(p, value, 0);
+        return write_string_file(p, value, WRITE_STRING_FILE_DISABLE_BUFFER);
 }
 
 int sysctl_read(const char *property, char **content) {