ublk: move auto buffer register handling into one dedicated helper

ublk: move auto buffer register handling into one dedicated helper

Move check & clearing UBLK_IO_FLAG_AUTO_BUF_REG to
ublk_handle_auto_buf_reg(), also return buffer index from this helper.

Also move ublk_set_auto_buf_reg() to this single helper too.

Add ublk_config_io_buf() for setting up ublk io buffer, covers both
ublk buffer copy or auto buffer register.

Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250713143415.2857561-7-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>