dm bufio: reorder fields in dm_buffer structure

Reorder fields in dm_buffer structure to improve packing and reduce
structure size.  The compiler allocates 32-bit integer for field 'enum
data_mode', so change it to unsigned char.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>