tests: skip u_direct_io if losetup fails

tests: skip u_direct_io if losetup fails

This new test requires a loop device to run testing.  While it checks
for some "obvious" parameters that might cause the test to fail such as
not being root and no losetup executable, it doesn't actually check that
the losetup -a call succeeds.  This causes a test regression in my
package building container (where there is only a minimal /dev with no
loop devices available) so I can't build debian packages.

Fix the test to skip out if we can't create a loop device.

Signed-off-by: Darrick J. Wong <djwong@kernel.org>
Signed-off-by: Theodore Ts'o <tytso@mit.edu>