ublk: handle ublk_set_auto_buf_reg() failure correctly in ublk_fetch()

ublk: handle ublk_set_auto_buf_reg() failure correctly in ublk_fetch()

If ublk_set_auto_buf_reg() fails, we need to unlock and return,
otherwise `ub->mutex` is leaked.

Fixes: 99c1e4eb6a3f ("ublk: register buffer to local io_uring with provided buf index via UBLK_F_AUTO_BUF_REG")
Reported-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Link: https://lore.kernel.org/r/20250521025502.71041-2-ming.lei@redhat.com
Reviewed-by: Caleb Sander Mateos <csander@purestorage.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>