SUNRPC: minor optimization of "xid" field in rpc_xprt

Move the xid field in the rpc_xprt structure to be in the same cache line
as the reserve_lock, since these are used at the same time.

Test plan:
None.

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