target/user: Introduce data_bitmap, replace data_length/data_head/data_tail

The data_bitmap was introduced to support asynchornization accessing of
data area.

We divide mailbox data area into blocks, and use data_bitmap to track the
usage of data area. All the new command's data would start with a new block,
and may left unusable space after it end. But it's easy to track using
data_bitmap.

Now we can allocate data area for asynchronization accessing from userspace,
since we can track the allocation using data_bitmap. The userspace part would
be the same as Maxim's previous asynchronized implementation.
Signed-off-by: Sheng Yang <sheng@yasker.org>
Signed-off-by: Nicholas Bellinger <nab@linux-iscsi.org>