nvmf: use io unit size in transport buffer pools

When this structure was brought up to the generic layer, the tcp
transport was using max_io_size and the rdma transport was using
io_unit_size. In the interest of conserving memory, we should use
io_unit_size instead of max_io_size.

Change-Id: I2633306fcbfd8c3d557445959c745cb2d9a0999e
Signed-off-by: Seth Howell <seth.howell@intel.com>
Reviewed-on: https://review.gerrithub.io/c/442778


Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>
Reviewed-by: Shuhei Matsumoto <shuhei.matsumoto.xt@hitachi.com>
Reviewed-by: Paul Luse <paul.e.luse@intel.com>
Reviewed-by: Ben Walker <benjamin.walker@intel.com>