svcrdma: Remove svc_rdma_marshal.c

svc_rdma_marshal.c has one remaining exported function --
svc_rdma_xdr_decode_req -- and it has a single call site. Take
the same approach as the sendto path, and move this function
into the source file where it is called.

This is a refactoring change only.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: J. Bruce Fields <bfields@redhat.com>