xprtrdma: Clean up synopsis of rpcrdma_regbuf_alloc()

Currently all rpcrdma_regbuf_alloc() call sites pass the same value
as their third argument. That argument can therefore be eliminated.

Signed-off-by: Chuck Lever <chuck.lever@oracle.com>
Signed-off-by: Anna Schumaker <Anna.Schumaker@Netapp.com>