SUNRPC: Rename automatic variables in unwrap_integ_data()

Clean up: To help orient readers, name the stack variables to match
the XDR field names.

For readability, I'm also going to rename the unwrap and wrap
functions in a consistent manner, starting with unwrap_integ_data().

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>