block: Inherit opt_transfer_length

block: Inherit opt_transfer_length

When there is a format driver between the backend, it's not guaranteed
that exposing the opt_transfer_length for the format driver results in
the optimal requests (because of fragmentation etc.), but it can't make
things worse, so let's just do it.

Signed-off-by: Kevin Wolf <[email protected]>
Reviewed-by: Wenchao Xia <[email protected]>
Reviewed-by: Max Reitz <[email protected]>
Reviewed-by: Benoît Canet <[email protected]>