svcrdma: Clean up RPC-over-RDMA Call header decoder

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

Rename the "decode" functions. Nothing is decoded here, they
perform only transport header sanity checking. Use existing XDR
naming conventions to help readability.

Straight-line the hot path:
 - relocate the dprintk call sites out of line
 - remove unnecessary byte-swapping
 - reduce count of conditional branches

Deprecate RDMA_MSGP. It's not properly spec'd by RFC5666, and
therefore never used by any V1 client.

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>