losetup: fix -f on systems without /dev/loop-control

Reported-by: Ruediger Meier <sweet_f_a@gmx.de>
Signed-off-by: Karel Zak <kzak@redhat.com>
Merge branch 'tests-known' of https://github.com/rudimeier/util-linux

* 'tests-known' of https://github.com/rudimeier/util-linux:
  tests, script/race is known to fail
  tests: script/race would work with variable loop count
  tests: ipcs/limits2 is known to fail
  tests: fix hexdump for big endian
  tests, add missing output for hexdump
  tests: fix hexdump, add ts_finalize
  tests: fix "KNOWN FAILED" for subtests
Merge branch 'master' into travis-losetup