SUNRPC: Rename svc_sock::sk_reclen

Clean up. I find the name of the svc_sock::sk_reclen field
confusing, so I've changed it to better reflect its function. This
field is not read directly to get the record length. Rather, it is
a buffer containing a record marker that needs to be decoded.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>