xprtrdma: Clean up XDR decoding in rpcrdma_update_granted_credits()

Clean up: Replace C-structure based XDR decoding for consistency
with other areas.

struct rpcrdma_rep is rearranged slightly so that the relevant fields
are in cache when the Receive completion handler is invoked.

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