svcrdma: Clean up rdma_build_arg_xdr

Clean up: The value of the byte_count parameter is already passed
to rdma_build_arg_xdr as part of the svc_rdma_op_ctxt structure.

Further, without the parameter called "byte_count" there is no need
to have the abbreviated "bc" automatic variable. "bc" can now be
called something more intuitive.

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