dm-error: add functions to create dm-error device

dm-error: add functions to create dm-error device

Controlled EIO from the device is achieved using the dm device.
Helper functions are at common/dmerror.

Broadly steps will include calling _dmerror_init().
_dmerror_init() will use SCRATCH_DEV to create dm linear device and assign
DMERROR_DEV to /dev/mapper/error-test.

When test script is ready to get EIO, the test cases can call
_dmerror_load_table() which then it will load the dm error.
so that reading DMERROR_DEV will cause EIO. After the test case is
complete, cleanup must be done by calling _dmerror_cleanup().

Signed-off-by: Anand Jain <anand.jain@oracle.com>
Reviewed-by: Filipe Manana <fdmanana@suse.com>
Reviewed-by: Dave Chinner <dchinner@redhat.com>
Signed-off-by: Dave Chinner <david@fromorbit.com>