ublk: rename auto buffer registration helpers

ublk: rename auto buffer registration helpers

Rename the auto buffer registration functions for clarity:

- __ublk_do_auto_buf_reg()    -> ublk_auto_buf_register()
- ublk_prep_auto_buf_reg_io() -> ublk_auto_buf_io_setup()
- ublk_do_auto_buf_reg()      -> ublk_auto_buf_dispatch()

Add comments documenting the locking requirements for each function.

No functional change.

Signed-off-by: Ming Lei <ming.lei@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>