test/nvmf: Increase size of the malloc bdev

xfsprogs refuse to create fs smaller than 300MB since:

https://git.kernel.org/pub/scm/fs/xfs/xfsprogs-dev.git/commit/?id=6e0ed3d19



To make sure mkfs does not fail, increase bdev size to 512MB.

Change-Id: Ic313db06cfab43f13f43a269c9cc058391bfb8ad
Signed-off-by: Michal Berger <michal.berger@intel.com>
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/17214


Reviewed-by: Konrad Sztyber <konrad.sztyber@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>
Reviewed-by: Jim Harris <james.r.harris@intel.com>