common/rc: ext4 doesn't support O_DIRECT with encryption

common/rc: ext4 doesn't support O_DIRECT with encryption

Teach _require_odirect to skip the O_DIRECT tests if the ext4 mount
option "test_dummy_encryption" is enabled, since O_DIRECT doesen't
make sense when file system level encryption is enabled, and several
tests which use O_DIRECT --- ext4/301, ext4/304, generic/250 and
generic/252 --- fail when encryption is enabled.

Signed-off-by: Theodore Ts'o <tytso@mit.edu>
Reviewed-by: Eryu Guan <eguan@redhat.com>
Signed-off-by: Eryu Guan <eguan@redhat.com>