tree-wide: fix write_string_file() user that should not create files

tree-wide: fix write_string_file() user that should not create files

The latest consolidation cleanup of write_string_file() revealed some users
of that helper which should have used write_string_file_no_create() in the
past but didn't. Basically, all existing users that write to files in /sys
and /proc should not expect to write to a file which is not yet existant.