SUNRPC: Fix a regression when reconnecting

If the task needs to give up the socket lock in order to allow a
reconnect to occur, then it must also clear the 'rq_bytes_sent' field
so that when it retransmits, it knows to start from the beginning.

Fixes: 718ba5b8 ("SUNRPC: Add helpers to prevent socket create from racing")
Signed-off-by: Trond Myklebust <trond.myklebust@primarydata.com>