loop: use LOOP_CONFIGURE instead of LOOP_SET_FD

LOOP_CONFIGURE allows to atomically configure the decive when opening
it. This avoid the possibility of a race condition where between set_fd
and set_status some operations are already accepted by the loopback
device. See https://lwn.net/Articles/820408/

This feature was included in the linux kernel 5.8 however it is safe to
not include any kind of fallback to the previous method as @obudai
points out that:

LOOP_CONFIGURE was backported into RHEL 8 kernel in RHEL 8.4 as a part
of https://bugzilla.redhat.com/show_bug.cgi?id=1881760 (block layer:
update to upstream v5.8).

Since RHEL 8.4 is currently the oldest supported release that we support
running osbuild on, it might be just fine implementing this without the
fallback.

From a centos stream 8 container:
kernel-4.18.0-448.el8.x86_64
- loop: Fix missing discard support when using LOOP_CONFIGURE (Ming Lei) [1997338]
- [block] loop: Set correct device size when using LOOP_CONFIGURE (Ming Lei) [1881760]
- [block] loop: unset GENHD_FL_NO_PART_SCAN on LOOP_CONFIGURE (Ming Lei) [1881760]
- [block] loop: Add LOOP_CONFIGURE ioctl (Ming Lei) [1881760]

test/run/test_devices.py

@@ -67,7 +67,7 @@ def test_loopback_basic(tmpdir):
         with pytest.raises(OSError):
             with open(testfile, "wb") as f:
                 f.truncate(1)
-                lo.set_fd(f.fileno())
+                lo.configure(f.fileno())
 
         lo.close()