mctp-bench: Support recv requests

"mctp-bench recv" can now optionally request that a peer start sending
with a specified message count and message length.

"mctp-bench recv eid 20 len 987 count 10000"

The send equivalent is not yet implemented. Currently mctp-bench will
continue listening forever, since it won't necessarily know that all
packets have been received (if drops occur). That could be changed in
future.

With default arguments "mctp-bench recv" behaviour is unchanged.

Signed-off-by: Matt Johnston <matt@codeconstruct.com.au>

Author: mkj