resctrl: Do not open directory for writing

resctrl: Do not open directory for writing

When preparing for the removal of GNULIB commit 18dca21a32e9 removed the
unneeded O_DIRECTORY, but unfortunately started opening the directory for
writing which fails every time for a directory.  There is also no need for that
as flock() works on O_RDONLY file descriptor as well, even for LOCK_EX.

https://bugzilla.redhat.com/1852741

Signed-off-by: Martin Kletzander <mkletzan@redhat.com>
Reviewed-by: Ján Tomko <jtomko@redhat.com>
Reviewed-by: Andrea Bolognani <abologna@redhat.com>