flexfiles: make rsize/wsize configurable

The read/write buffer size between a and the data server is offered by
MDS. Currently, this value is hard coded to 64k, which is not optimal
value for modern hardware.

Allow mds to specify preferred IO size for DS. Though this value can be
different for each data server, we use a single buffer size by assuming
that in typical installation data servers have a similar IO

As Linux client uses single value for rsize and wsize, we use (for
pragmatic reasons) a single value as well.

The IO size between clients and the server can be configured.

Acked-by: Paul Millar
Target: master
1 job for master in 1 minute and 10 seconds (queued for 15 seconds)
Status Name Job ID Coverage
passed build #980