common: Mark tests which use direct I/O with _requires_odirect

common: Mark tests which use direct I/O with _requires_odirect

There were a number of tests that use Direct I/O that weren't testing
to make sure O_DIRECT is actually supported.  This will be important
for avoiding false positives when testing ext4 encryption (which does
not support DIO for obvious reasons).

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>