test/nvmf: increase malloc size in filesystem test

btrfs.tools v5.15.1 in Fedora 35 complains about target
device being too small to create a filesystem.
Let's increase the bdev size to this test.

Signed-off-by: Karol Latecki <karol.latecki@intel.com>
Change-Id: Iedf59328b91516693c88af8c5d47dde849394337
Reviewed-on: https://review.spdk.io/gerrit/c/spdk/spdk/+/10614


Reviewed-by: Michal Berger <michalx.berger@intel.com>
Reviewed-by: Pawel Piatek <pawelx.piatek@intel.com>
Reviewed-by: Maciej Wawryk <maciejx.wawryk@intel.com>
Reviewed-by: Changpeng Liu <changpeng.liu@intel.com>
Reviewed-by: Tomasz Zawadzki <tomasz.zawadzki@intel.com>
Tested-by: SPDK CI Jenkins <sys_sgci@intel.com>