svcrdma: Clean up backchannel send header encoding

Replace C structure-based XDR decoding with pointer arithmetic.
Pointer arithmetic is considered more portable.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>