MADs are UD and can be dropped if there are no receives posted, so
allow receive queue size to be set with a module parameter in case the
queue needs to be lengthened.  Send side tuning is done for symmetry
with receive.
Signed-off-by: Hal Rosenstock <hal.rosenstock@gmail.com>
Signed-off-by: Roland Dreier <rolandd@cisco.com>