SUNRPC: Refactor xprt_transmit() to remove wait for reply code

Allow the caller in clnt.c to call into the code to wait for a reply
after calling xprt_transmit(). Again, the reason is that the backchannel
code does not need this functionality.

Signed-off-by: Trond Myklebust <trond.myklebust@hammerspace.com>