xprtrdma: Simplify synopsis of rpcrdma_buffer_create()

Clean up: There is one call site for rpcrdma_buffer_create(). All of
the arguments there are fields of an rpcrdma_xprt.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Reviewed-by: Steve Wise <swise@opengridcomputing.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>