[PATCH] nbd: Permit nbd-client to set minimum and optimal I/O sizes

["Richard W.M. Jones" <rjones@redhat.com>]

This is a small change to the nbd kernel driver which allows the
client to set minimum_io_size and optimal_io_size hints for the NBD
device.

There is an accompanying patch for nbd-client (userspace tool) which I
will send separately.

Rich.